Bureau of StaaftArdd
FED 4 1937
U. S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS
INKS
CIRCULAR C413
U. S. DEPARTMENT OF COMMERCE
DANIEL C. ROPER, Secretary
NATIONAL BUREAU OF STANDARDS
LYMAN J. BRIGGS, Director
CIRCULAR OF THE NATIONAL BUREAU OF STANDARDS C413
INKS
By C. E. WATERS
[Issued December 28, 1936. Supersedes C400]
UNITED STATES
GOVERNMENT PRINTING OFFICE
WASHINGTON : 1936
For sale by the Superintendent of Documents, V/asliington, D. C.
Price 10 cents
PREFACE
Since 1906, when the National Bureau of Standards began to test
writing inks and a few other kinds, many hundreds of letters have
been received from persons who wanted all sorts of information about
inks. A printed circular can give a great deal more than would be
feasible in a letter, so in 1920 Circular C95, Inks — Their CoKiposition
and Manufacture, was published. The second edition, in 1925, was
called Inks, Typewriter Ribbons, and Carbon Paper. These gave
almost no formulas, and because so many letters asked how to make
inks. Circular C400, Inks, was written to try to satisfy this demand.
Fifty or more formulas for a variety of inks were given in it. It was
issued at the end of 1932 and since then has been in great demand,
about 2,600 copies having been distributed.
The present Circular is a thoroughly revised and enlarged edition
of Circular C400. It contains new formulas, and discusses subjects
that were only briefly touched upon, if at all, before. It is believed
that the present Circular is a distinct improvement on the one it
supersedes.
Lyman J. Beiggs, Director.
II
INKS
By C. E. Waters
ABSTRACT
This circular outlines briefly the history of writing inks, in particular those
of the iron gallotannate type, gives formulas for a few of these inks and for three
new iron gallate inks, discusses the aging of writing, the restoration of faded
writing, and the effect of writing inks upon paper. After this, come brief dis¬
cussions of several other kinds of inks, including colored writing inks, drawing,
stamp-pad, recording, and other kinds. Formulas are given for most of them.
Printing inks and others that depend upon pigments for their color and their
special properties are in a class by themselves, and little is said about them in
this circular.
The methods of testing given in the Federal specifications for inks are de¬
scribed. Then follows an appendix in which are sections on weights and measures,
on equipment for making ink in the home, and on dyes suitable for making
inks. Finally, there is given a brief list of selected references.
CONTENTS
Page
Preface - ii
I. Introduction _ 2
II. Iron gallotannate and gallate inks _ 3
1. Ancient inks _ 3
2. Modern inks _ 4
3. A definition of ink _ 4
4. The tannin in iron gallotannate inks _ 5
5. Formulas for iron gallotannate and gallate inks _ 7
(a) History of formula for Government record ink _ 7
(b) Standard for Government copying and record ink _ 8
(c) Former standard for Government copying ink _ 9
(d) Standard for Government writing ink _ 9
(1) Concentrated ink _ 10
(2) Ink powders and tablets _ 10
(e) Iron gallate inks _ 10
6. Preparation of iron gallotannate and gallate inks _ 12
7. Ammonium ammoniumoxyferrigallate ink _ 14
8. Aging of writing _ 14
9. Dating a document _ 16
10. Restoration of faded writing _ 19
11. Effect of writing ink upon paper _ 20
12. Ink eradicators _ 21
III. Other kinds of inks _ 22
1. Carbon inks _ 22
(a) Carbon writing and drawing inks _ 22
(b) Printing, canceling, and other carbon inks _ 23
2. Dye inks for writing _ 24
(a) Washable inks _ 25
(b) Quick-drying inks _ 25
3. Prussian blue inks _ 26
4. Colored drawing inks _ 28
5. Show-card inks _ 30
6. Hectograph inks _ 31
7. Stamp-pad inks _ 32
8. Recording inks _ 33
1
2 Circular of the National Bureau of Standards
III. Other kinds of inks — Continued. Page
9. Indelible marking ink for fabrics _ 35
10. Sympathetic or invisible inks _ 36
11. Inks for special surfaces _ 38
(a) Inks for celluloid _ 38
(b) Inks for glass and porcelain _ 39
(c) Etching inks for glass _ 40
(d) Ink for zinc garden labels _ 40
(e) Ink for brass _ 40
(f) Ink for other metals _ 41
(g) Time-card ink _ 41
IV. The testing of inks _ 41
1. Iron gallotannate ink _ 41
2. Red ink _ 45
3. Stamp-pad ink _ 45
4. Indelible marking ink for fabrics _ 46
5. Black and colored drawing inks _ 46
V. Appendix _ 47
1. Weights and measures _ 47
2. Equipment for making ink _ 49
3. Dyes for making ink _ 49
4. Literature on inks _ 52
I. INTRODUCTION
Nobody can say how early in his long history man began to use
signs and symbols to serve as reminders to himself, and to convey
information to his fellows. No doubt the earliest of such signs were
piles of stone, and the broken twigs we still use to mark an unfamiliar
trail. The spirited though crude drawings left on the walls of Euro¬
pean caves by men of earlier cultures than ours show that primitive
man was akin to us. Worse art is to be seen today on walls in wait¬
ing rooms and other public places.
The walls of caves, fiat rocks on the faces of cliffs, clay tablets,
smooth slabs of wood, sheets of wax, and pieces of ivory, bone, and
skin have all been used for v/riting upon. Even today a college
diploma is a sheepsldn in name if not in fact, and tattooing has not
died out. For centuries parchment — the better kind is called vellum
— was the material on which many books were written, and the papy¬
rus roll was common enough to have given us the word ‘‘paper.
Parchment and papyrus were expensive, and could not be obtained
in large quantities, and there could have been no great development
of printing, nor much letter writing, if the art of making paper from
pulped vegetable fibers had not been invented.
We may never know v/hen writing ink was first used, nor what it
was made of. No doubt the juices of colored berries served as ink
at a very early date, but it would be hopeless to look for samples of
writing done with them. Dyes, whether made by nature or by man,
have the unfortunate habit of fading. The same colored pigments
that v/ere stirred up with water to make war paint might also have
been used for writing. The ink on the oldest manuscripts that have
been found, which date from about 2500 B.C., was made with carbon,
probably in the form of lampblack (soot) in most cases, though char¬
coal may have been used also. It is not known whether the lamp¬
black was merely stirred with water and kept in this liquid form, or
v/hether it was made into the sort of dry cakes we call Chinese, or
more often india, ink. These cakes, which have been in use for
Inks
3
3,000 years, are prepared by making lampblack into a stiff paste
with a solution of glue, gelatin or ^^gum”, (possibly gum arabic) in
water, shaping the mass in molds, and drying it. Wlien some ink is
needed for writing, the end of a cake is rubbed with a little water in a
shallow dish until enough of the dry ink to give the desired depth of
color is dissolved.
Another much used kind of ink was sepia, a dark-brown secretion
from cuttlefish, the same kind of animal that serves as food for man,
in Europe, and provides the cuttle bone which hangs in the canary’s
cage.
Those who wish to read more about the history of vTiting are
advised to consult books in public libraries. Two books that are
suggested are by Mitchell and Hepworth,^ and by Carvalho.^
II. IRON GALLOTANNATE AND GALLATE INKS
1. ANCIENT INKS
Leather tanned with bark was knovm before the Christian Era,
and the staining of wet leather by contact with iron must have been
noticed often. Yet the world waited for more than 2,000 years after
the invention of india ink, or until about 1126 A.D., before tannin
and iron were combined to make writing ink. This kind of ink is
still used in larger quantities than any other. The ink was made by
dissolving ferrous sulphate (copperas or green vitriol) and glue or
gum in an infusion of nutgalls, which contain a kind of tannin that
is especially suitable for making ink. The infusion of nutgalls was
allowed to ferment, the other materials were added, and the mixture
was left undisturbed for a timie, so that solid impurities could settle
out, and the solution could blacken. The change in color was caused
by the action of oxygen from the air upon the iron salt. Ferrous
iron forms vdth tannin an easily soluble compound that is not intensely
colored, and oxygen converts this more or less completely into a
ferric compound, which is black, and is nearly insoluble in water.
Ink made by this process was a muddy fluid in which floated innumer¬
able microscopic particles of the black compound. The glue or the
plant gum helped to keep the particles from settling to the bottom
of the fluid, and later served to fasten them to the paper or parchment.
In the early days there was no thought of chemical control of the
manufacturing process, nor any chemist who could have supervised
it. Not until 1748, when William Lewis began to experiment, was
any attempt made to produce a “balanced” ink, with nearly correct
proportions of iron and nutgalls; and even in his time there were no
analytical methods to help him. Though he had to work by the
cut-and-try method, he tried.
Because each ink maker used the formula he considered the best,
but had no idea of the amount of tannin in the galls, nor of the purity
of his ferrous sulphate, many a batch of ink must have been far from
balanced in chemical composition. This state of affairs is reflected in
the condition of old documents preserved in libraries in Europe.
More about this will be found on page 20.
1 C. A. Mitchell and T. C. Hepworth, Inks, Their Composition and Manufacture, 3d ed. (Chas. Griffin
& Co. (Ltd.), London, 1924.)
2 D. N. Corvalho, Forty Centuries of Ink. (The Banks Law Publishing Co., New York, 1904.)
4
Circvlar oj the National Bureau of Standards
2. MODERN INKS
About the middle of the nineteenth centur}^ a change was made in
the manufacture of writing ink. Instead of deliberately letting it
oxidize and be turned into a muddy fluid, it was guarded from the
action of the air and kept clear as long as possible. When a batch of
ink is made nowadays, it is allowed to remain undisturbed for a time
so that solid impurities will settle to the bottom, but only a small part
of the iron salts in a vat containing some hundreds of gallons of ink
will become oxidized.
The coloring matter of the older inks consisted of black particles
that remained to a great extent upon the surface of the paper. The
modern clear inks soak into the fibers of the paper, or penetrate
betyreen them, and then become oxidized. For this reason it can be
argued that the clear inks should be the more permanent, because so
little of the writing is on the surface, where it can be rubbed off. To
keep the ink clear as long as possible, it must be kept from oxidation,
and must also contain a small quantity of free acid, usually hydro¬
chloric or sulphuric acid, to hold in solution the black iron compound
whose formation can not be avoided entirely. The more free acid the
ink contains, the longer will it remain clear, but the greater will be
its destructive effect upon paper, and its corrosive action on steel pens.
There must be some sort of compromise if the use of iron gallotannate
ink is not to be abandoned. Our ancestors tvro or three generations
back were not concerned with the acidity of their ink. It was muddy
anyhovv', and they had no steel pens to be corroded. The fountain
pen with its noncorrodible point puts a temptation in the path of the
ink maker, vrho knows what an extra amount of acid will do for him
in keeping the ink clear.
Ink which has undergone but httle oxidation does not look intenseW
black in the bottle, and the marks it makes on paper are so pale at
first that it is necessary to give the ink a stronger color by the addi¬
tion of a dye. The dye would not be needed if letters could always be
kept for a day or tvro for them to become easier to read. A, Tien clear
gallotannate inks began to be made, the synthetic or so-called aniline
dyes were something yet to be discovered. Of the com.paratively few
dyes available in those days, it is probable that only indigo could have
been used without causing the precipitation of solid matter in the ink.
Indigo itself is not soluble, but by suitable treatment with strong sul¬
phuric acid it is converted into the disulphonic acid, which dissolves
readily and forms no precipitate by combining vith the other ingre¬
dients of the ink.
3. A DEFINITION OF INK
In 1890, Schluttig and Neumann, ink chemists of Dresden, Ger¬
many, wrote what is in many respects the most important book on
iron gallotannate inks, because of its far-reaching and lasting in¬
fluence.^ Their definition of ink, their explicit recommendations for
making record ink, and the whole tone and spirit of the book set a new
mark for the ink maker to ahn at. As a basis for some of the dis¬
cussion in the pages which follow, their definition, in nearly literal
translation, is here given.
* O. Schluttig, and G. S. Neumann, Die Eisengallustinten (The Iron-Gall Inks), (v. Zahn d: Jaensch,
Dresden, 1890).
Inks
5
By ink we mean a liquid, suitable for writing, which
1. Is a clear, filterable solution, not a suspension;
2. Is mobile and keeps for a considerable time; that is, it flows easily from the
pen, and neither clogs, drops off, nor spreads on the paper;
3. Has good keeping quality in glass; that is, in the inkstand it forms
(a) A slight deposit only slowly,
(b) No skin-like deposit, on the surface or on the walls, and never any mold;
4. On a good pen it forms only a slight, varnish-like, smooth coating, but not a
loose, crusted one;
5. Has no pronounced odor;
6. Is not too acid and does not penetrate through good paper;
7. Has an intense color, which does not become paler nor bleach out entirely
in the liquid or on paper (in the latter case Judged after the complete drying of the
writing, for moist lines always look darker than dry ones) ;
8. Gives writing that is not sticky after drying.
Every good ink, whether writing, or combined writing and copying,
should have these qualities. There is no sharp boundary between the
two kinds, but if the ink is intended only for writing, it should in
addition:
9. Give writing that, after drying for eight days, is not removed by water or
alcohol — even by treatment for days — to such an extent that it becomes illegible.
Finally, if the ink is intended for imperishable records, it must have:
10. A definite minimum content of iron,
11. And enough tannin; that is, it must give writing which after drying becomes
deep black within eight days, and which, even after treatment for days with water
and alcohol, still retains a certain degree of blackness.
All the points in this curious definition are important, but not
equally so. The authors stressed 1, 3, 6, 10, and 11. No. 10, of
course, means not less than a definite minimum content of iron. In
the work described in their book they tried to make ink that met the
requirements of their definition to the fullest possible degree. Taking
it for granted that the ink should contain iron, they first studied the
effect of having different amounts of that metal and of gallic and
tannic acids in the solution. Having done their best with these ma¬
terials, they extended their investigation to include inks made with
iron and substances that are closely related to gallic acid in their
chemical structure. Their conclusions may be better understood
after reading a short discussion of galhc and tannic acids.
4. THE TANNIN IN IRON GALLOTANNATE INKS
The tannins are a group of more or less closely related chemical
compounds that are found in many different kinds of plants. Their
name comes from the use of some of them for tanning the skins of
animals to make leather. The chemistry of this group of substances
is quite complicated, but a good start has been made in determining
the molecular structure of the tannins. It has been proved that
many of them are glucosides, or compounds of the familiar sugar,
glucose (dextrose), with the various organic acids that are the real
tanning agents. Chemically, glucose is an alcohol, and its compounds
with these acids are esters, or salts, as trul^ as ethyl acetate is the
ester, or salt, of ethyl alcohol and acetic acid. Under suitable con¬
ditions, ethyl alcohol and acetic acid react as shown by the following
equation, to produce ethyl acetate and water. The atoms that pre¬
sumably unite to form water are in italics.
CH8C0.0H-b(H0)C2H6 = CH3C0.0C2H5-fH20
acetio ethyl ethyl water
acid alcohol acetate
6
Circular of the National Bureau of Standards
If ethyl acetate is heated with water, the reverse reaction takes
place to some extent, and ethyl alcohol and acetic acid are recovered,
as shown by reading the equation from right to left. The ethyl
acetate can be broken down completely into alcohol and acid if some
caustic soda is dissolved in the water, but in thus case sodium acetate,
and not the free acid, is obtained. This is a typical example of
‘ ‘hydrolysis’ h or the splitting of a compound by its combining chemi¬
cally with water. By an exactly similar reaction the natural tannins
that are glucosides can be split into glucose and organic acids.
As already said, glucose, C6H12O6, is an alcohol, but it differs from
ethyl alcohol in being able to combine with five molecules of acid,
instead of with only one. This difference can be indicated by writing
the formula of glucose thus: (H0)5C6H70. Then if KCO.OH repre¬
sents any one of the numerous acids that occur in the tannins, its
glucoside may perhaps be formed in the plant by the following reac¬
tion, which has been brought about in the chemical laboratory:
5RC0.0H+(H0),C,B.70=^{RC0.0),C,-R^0 + 5B.20
“tannic” glucose glucoside water
acid
As before, the atoms that unite to form water are in itahcs. The
equation when read from right to left expresses the hydrolysis of the
glucoside into acid and glucose.
The best known of the natural tannins is the glucoside of tannic
acid, which is also called gallotannic acid, digallic acid, gaffylgallic
acid, or simplj tannin. Three of these indicate a relationship to
gallic acid, which gets its name from the ultimate source, nutgalls.
To the organic chemist, galhc acid is 3, 4, 5-trihydroxybenzoic
acid, which means that it is benzoic acid, CsHsCO.OH, in which the
hydrogen atoms in the 3, 4, and 5 positions with respect to the car¬
boxyl group, CO. OH, are replaced by hydroxyl, OH. Because it is
a derivative of benzene, the structural formula of gallic acid is
HO.C _ CH
HO.C<(^ ^C.CO.OH.
HO.C CH
The formula can also for convenience be written as (HO)3C6H2CO.-
OH, or for the present discussion as (HO)(HO)2C6H2CO.OH. Gallic
acid is thus both an acid, on account of the carboxyl, and an alcohol,
because of the hydroxyl groups. Just as ethyl alcohol and acetic
acid combine to form an ester, so two molecules of gaUic acid can
react to form an ester, in this case tannic acid. The equation makes
this clear:
HO.C
HO.<
CH HO.C
\c.CO.OH + HO.c/
CH
")>C.CO.OH= H2O
^ water
+
HO.C CH
Two molecules of gallic acid
HO.C CH
Inks
7
HO.C _ CH
liO.C<^ ^C.CO OC CPI
HO.C CH HO.c/ Nc.CO.OH
HO.C CH
Tannic acid
It will be seen that tannic acid has a carboxyl group, so it can form
salts or esters, including those known as glucosides. Finally, be¬
cause it is an ester, it can be hydrolyzed in the same way as ethyl
acetate. The equation just given, when read from right to left,
shows how tannic acid takes up water and is hydrolyzed into two
molecules of gallic acid.
Return now to the last two sentences of section 3. In addition
to critically studying inks containing different amounts and propor¬
tions of gallic and tannic acids, Schluttig and Neumann made inks
with 26 other substances that are chemically related to these two acids,
but differ from them in the number and arrangement of the hydroxyl
and carboxyl groups attached to the benzene ring, or that have
methoxyl, OCH3, groups instead of hydroxyl. Their conclusion was
that in order to make ink of good color and permanence, the ‘ 'tannin’’
must have three adjacent, free hydroxyl groups. This condition is
satisfied by tannic acid and gallic acids, and these of all the substances
studied were the best for making ink. Thus a formula probably
discovered by accident, and improved empirically during the cen¬
turies, was shown to be scientifically correct, so far as an essential
part of it is concerned.
5. FORMULAS FOR IRON GALLOTANNATE AND GALLATE INKS
(a) HISTORY OF FORMULA FOR GOVERNMENT RECORD INK
It does not suffice to find the best materials for making ink, because
unless they are used in the correct amounts, the ink will not be good.
There should be no excess of either iron salt or of tannin, the amount
of free mineral acid should be just enough to keep the ink clear for a
reasonably long time in the bottle, and there must not be a deficiency
nor an excess of dye. The formula for "copying and record” ink
given on page 9 of this circular differs in but two respects from the
one recommended by Schluttig and Neumann, as the result of their
long investigation. They used 10 grams (hereafter written "g”)
of gum arabic, the "acacia” of the U. S. Pharmacopoeia, in a liter of
ink, to act as a preservative, as they put it. As we would now say,
it served as a "protective colloid” to hinder the precipitation of any
insoluble ferric gallotannate formed in the ink. Some years ago the
gum was omitted from the United States Government formula when
proposals for bids were being typewritten. This clerical error was
discovered too late to be corrected, because the contract for a year’s
supply of ink had been awarded. This led the then Bureau of Chemis¬
try, U. S. Department of Agriculture, to make special tests of the
ink v/ith and without gum arabic. These tests showed that the
omission of the gum was an improvement, and ever since then the
gum has been left out of the formula for the standard ink.
101251°— 36 - 2
8
Circular oj the National Bureau oj Standards
The Government formula calls for a dye that is different from the
one recommended by Schluttig and Neumann, though the two are
closely related in comiposition. This is further discussed on page 51.
In the early 1890’s, the Commonwealth of ]Massachn setts adopted
the Scliluttig and Neumann formula as the official ink for records
and other public documents. A few years later the Federal Govern¬
ment started to use it, and about 1914 Connecticut followed their
lead. It should be noted that Prussia, in 1912, decreed that the
official ink should contain at least 4, and not more than 6, g of iron in
a liter, these being the limits set by Schluttig and Neumann for
record ink. In tiiis country the maximum, 6 g of iron, has always
been required. This weight of the metal is contained in the 30 g of
ferrous sulphate crystals in the formula. Massachusetts requires
10 g of gum arabic in a liter of ink, but curiously enough is silent
about the use of blue dye.
(b) STANDARD FOR GOVERNMENT COPYING AND RECORD INK
In 1924, the Federal Specifications Board took over the old speci¬
fication for “Treasury Standard’’ writing ink, and promulgated it as
United States Government Master Specification 163, Pecord and
Copying Ink. It was published as Circular Cl 82 of the National
Bureau of Standards, wliich went out of piint in 1930. iUthough it
is not a true copying ink, it viU give one good press cop}^ when the
writing is fresh, and tins general^ suffices. In 1930, the specifica¬
tion was changed in form, but not in technical requirements, and
issued as Federal Specification TT-I-521, Ink; Copying and Record,
a part of the Federal Stock Catalog. The apparent subordination of
“record” to “copying” is the result of alphabetical exigencies, so that
the specification will fit into its proper place in the catalog.
Like other specifications for inks, this one gives a formula for
making ink to be used as a standard for comparison when testing
samples of inks bought by the Government. I\lany who read the
specification labor under the mistaken impression that the manu¬
facturer must use the same pure materials for producing the ink he
furnishes. This idea is not m accord with the vcorcling or the intent
of the specification. It is necessary, in order that both buyer and
seUer shall be able to test the ink on the same basis, that the standard
ink shall be made of chemicals of definite purity. Any manufacturer
who has the knowledge and skill to use cheaper raw materials in
making ink that meets the requirements of the specification, may
do so.
A standard ink is necessary because some of the requirements of
the specification can not be stated in exact terms, and because some
of the properties of the ink that are measured by the tests may differ
according to the conditions under which the tests are made. If the
standard and the sample are put through the same series of tests,
side by side, it is easy to see whether the sample is equal to the
standard in all essentials.
In the following formula for the standard copying and record ink
of Federal Specification TT-I-521, aU the materials must be “of the
strength and quality prescribed in the edition of the United States
Pharmacopoeia which is current at the tune bids are asked for.”
Tbk, however, does not apply to the dye.
Inks
9
Standard Comjing and Record Ink
Grams
Tannic acid _ 23. 4
Gallic acid crystals _ 7. 7
Ferrous sulphate crystals _ 30. 0
Hydrochloric acid, “dilute”, U.S.P _ 25. 0
Carbolic acid (phenol) _ 1. 0
Soluble blue (C.L 707; Sch. 539)^ _ 3. 5
Water to make a volume of 1 liter at 20° C (68° F).
* The symbols in parenthesis, here and throughout the circular, indicate definitely the dye type intended.
See explanation on p. 49.
The temperature at which the volume is made equal to 1 liter is,
for practical purposes, of no importance. This applies also to the
next two formulas.
(c) FORMER STANDARD FOR GOVERNMENT COPYING INK
There is no Federal specification for true copying ink. Years ago
fairly large quantities of “Treasury Standard” copying ink were
bought on a specification based on the following formula:
Standard copying ink
Tannic acid _
Gallic acid crystals _
Ferrous sulphate crystals _
Hydrochloric acid, “dilute”, U.S.P _
Gum arable (acacia, U.S.P.) _
Carbolic acid (phenol) _
Soluble blue dye ® _
Water to make a volume of 1 liter at 20° C (68° F).
» In those days, the dye was expected to be bavarian blue DSF, which was recommended by Schluttig
and Neumann. See also p. 52.
Grams
46. 8
15. 4
60. 0
50. 0
10. 0
1. 0
5. 0
As in the preceding formula, and in the one which follows, all the
ingredients except the dye are to be of the strength and quality pre¬
scribed in the current United States Pharmacopoeia.
To make the writing transfer more readily in a letterpress, some
copying inks are made with the further addition of dextrin, sugar,
glycerol (glycerin), or other similar substance. If too much is used,
the writing will be sticky.
(d) STANDARD FOR GOVERNMENT WRITING INK
The copying and writing ink is of too heavy a body to please most
writers, so there is a Federal Specification, TT-I-563, Ink; Writing.
It was written originally to provide ink for use in post-office lobbies,
where the conditions are devastating to pens. The standard ink of
this specification is similar to some of the commercial writing inks.
Except for the amounts of dye and preservative (carbolic acid), it is
half as concentrated as the copying and record ink. The effectiveness
of the preservative depends upon the quantity of it in a given volume
of solution, and the ink must contain only enough dye to give a good
color to the fresh writing. So the weights of these two ingredients are
the same as in the more concentrated ink. The formula for the
standard writing ink is:
Standard Writing Ink
Grams
Tannic acid _ 11. 7
Gallic acid crystals _ 3. 8
Ferrous sulphate crystals _ 15. 0
Hydrochloric acid, “dilute”, U.S.P _ 12. 5
Carbolic acid (phenol) _ 1. 0
Dye (C.I. 707; Sch. 539) _ 3. 5
Water to make a volume of 1 liter at 20° C (68° F).
10
Circular of the National Bureau of Standards
I
It is hardly necessary to say that the materials for making this ink
must be of the same quality as those for making the other standard
iron gallotannate inks.
(1) Concentrated Ink. — Concentrated ini-?: is accepted if it meets the
requirements of the specification for writing ink. The contents of
the usual small bottle or the collapsible tube in which the ink is packed
will make a quart of writing fluid when mixed with water. Hydro¬
chloric acid is a solution of a gas in water, and it is therefore volatile.
For this reason it is probable that most producers of concentrated
ink use an equivalent amount of sulphuric acid instead, because for
all practical purposes it is nonvolatile.
Concentrated ink occupies less space and weighs less than its
equivalent in writing fluid, so the bottle is not so apt to be broken
in shipment as a quart bottle of fluid, to say nothing of the saving in
express or postal charges. It is also less apt to freeze and burst the
bottles than the more dilute writing fluids; while if breakage should
occur, surrounding packages will suffer less harm.
(2) Ink Powders and Tablets.- — Ink powders and tablets represent
the last step in concentrating ink. At the time the Federal specifi¬
cation for writing ink was written, and for some years afterwards, the
few samples of these kinds of ink that had been examined by the
National Bureau of Standards consisted wholly of dyes, or else they
were quite unsatisfactory mixtures that purported to make good iron
gallotannate inks. It is needless to point out their faults in detail.
The chief problem to bo solved in making an ink powder — a tablet
is only the compressed powder — is to find a dry acid that will fully
take the place of hydrochloric or sulphuric acid. This problem was
solved in 1931 by one manufacturer, whose product has been tested
numerous times by the Bureau, though never completely analyzed.
Since then a chemist of the Bureau has developed two formulas for
ink powder, that are given in the next section.
(e) IRON GALLATE INKS
With the primary object of making an ink powder that will produce
writing fluid of good keeping quality, low acidity, and satisfactory
permanence, a great number and variety of formulas were systemati¬
cally studied by one of the chemists of the Bureau.® It was found that
ink v/ill keep longer without depositing sediment if it is made without
tannic acid, but with an increased amount of gallic acid. It was also
found possible to replace the usual hydrochloric or sulphuric acid
by less than an equivalent quantity of a solid organic acid. Two of
the formulas in this section will make ink powders that can be kept
in the dry state for a long time, and that produce unusually stable
writing fluid, provided dye of the right (juality is used. This is
discussed in detail further along in this section.
The first of the two formulas for ink powder is a departure from
custom, in that it requires ferric sulphate instead of ferrous sulphate.
The weights of the ingredients needed to make a liter of writing
fluid are:
Ferric Sulphate Ink Powder
Grams
Gallic acid crystals _ 10. 0
Ferric sulphate, anhydrous _ 10. 7
Oxalic acid crystals _ 2. 0
Soluble blue (C.I. 707; Sch. 539)) _ 3. 5
6 E. W. Zimmerman, Iron gallate inks— liquid and powder. J. Research NB3 15, 35-40 (1935) RP807.
Inks
11
The weight of ferric sulphate called for, 10.7 g, contains 3 g of iron.
An equivalent amount of the hydrous salt can be used instead. The
simplest plan is to determine the iron content of the ferric sulphate
and from this to calculate the weight of the salt that will give 3 g of
iron. Ink made by this formula does not corrode steel pens exces¬
sively, but it foyms on them a thin, yellowish coating of ferrous oxa¬
late.
A formula that makes a still better ink powder than the preceding
is:
Ferrous Sulphate Ink Powder
Grams
Gallic acid crystals _ 10. 0
Ferrous sulphate crystals _ 15. 0
Tartaric acid _ 1. 0
Soluble blue (C.I. 707; Sch. 539) _ 3. 5
As with the first formula, the ingredients are to be dissolved in
enough water to make a total volume of 1 liter, and this will contain
3 g of iron.^
It was pointed out in the first paragraph of this section that the
two formulas for powder will make ink that keeps well in the bottle,
provided dye of satisfactory quality is used. Certain adverse com¬
ments by manufacturers led to the suspicion that the premature de¬
position of sediment that they complained about might be caused
by the dye. Accordingly 10 lots of ink were made, by the second
formula, that differed only in that each contained soluble blue from
a different mianufacturer, or 2 grades from a single manufacturer.
During the 2 weeks of the usual sediment test (see p. 42), five of the
inks deposited from a slight amount to a great deal of sediment. The
other five inks stayed clear, and in this respect were superior to the
standard v/riting ink of the Federal specification. The formation of
sediment is caused chiefly by the action of atmospheric oxygen, so if
it is kept in a bottle that contains a great deal of air above it, ink v/ill
sometimes become turbid in a week or two. The more air and the
less ink in the bottle, the sooner will sediment be formed. A sample
of ink made by the second formula, and containing dye that was knowm
to be satisfactory for the standard ink of the specification, was kept
in a half-filled bottle for 8 months before any sediment could be seen
in it.
Strange to say, a solution that contains all the ingredients of the
second formula except the dye is less stable, and becomes cloudy
within a week. The reason for this is not known, but the actual fact
has been checked a number of times with materials from different
sources. It may be that the dye molecule itself acts as an antioxidant
and retards the formation of sediment; or that the presence or ab¬
sence of some impurity determines the rate of oxidation. On the
other hand it is possible that the dye or an impurity in it acts as a
‘ 'protective colloid”, and delays the flocculation and precipitation of
the colloidal particles of iron gallate in the solution.
The third of the new formulas does not make ink powder, because
in it sulphuric acid is used instead of tartaric acid. For this reason
it could be used for concentrated ink, especially because of the small
volume of concentrated sulphuric acid required. Although not so
? A patent based upon this formula has been applied for. If granted, it will be assigned to the U. 8.
Government.
12 Circular of the National Bureau of Standards
named in the original publication, the following formula may be
called:
Concentrated Writing Ink
Grams
Gallic acid crystals _ 10. 0
Ferrous sulphate crystals _ 15. 0
Sulphuric acid (calculated as anhydrous) _ 0. 654
Soluble blue (C.I. 707; Sch. 539) _ 3. 5
"Water to make a pasty mass or, for making writing fluid directly,
enough water to make the total volume 1 liter.
Anhydrous sulphuric acid is so nearly impossible to obtain and to
keep that 0.69 g (0.654/0.95) of the familiar 95-percent acid can be
substituted in the formula. One milliliter of acid of this concentra¬
tion weighs 1.84 g, so 0.69 g will be 0.37 ml. A liter of the standard
iron gallotannate waiting ink of the Federal specification contains 1.25
g of hydrochloric acid gas, equivalent to 1.77 g, or 0.96 ml, of 95-
percent sulphuric acid. The use of only 0.69 g of this corrosive acid
is greatly to the advantage of the new^ formula.
It is unfortunate that gallic acid is so little soluble that the formulas
in this section can not be modified for making record ink containing
the required 6 g of iron in a liter. On this basis 20 g of gallic acid
would have to be dissolved in a liter. At 25° C (77° F) a liter of
water will dissolve only 12.0 g of gallic acid, and still less at lower
temperatures. The presence of the iron salt defmitely increases the
solubility of gallic acid, but not sufficiently. It has been found that
at 25° C ink containing betw^oen 17 and 18 g of gallic acid in 1 liter
can be made.
6. PREPARATION OF IRON GALLOTANNATE AND GALLATE INKS
In the formulas already given, and elsewhere in this circular, when
w^ater is called for it is to be understood that distilled water is best,
with rainw^ater the second choice. There are parts of the country
where the water of streams and w-ells is so hard, because of the calcium
carbonate dissolved in it, that a substantial part of the acid in iron
gallotannate and gallate inks wall be neutralized if natural water is
used for making them. Water that contains 300 parts per million of
calcium, calculated as the carbonate, or 0.3 g in a liter, is not unkno^vn.
This quantity of ‘fiime” wall neutralize nearly 2.2 g, or almost one-
eleventh, of the 25.0 g of dilute (lO-percent) hydrochloric acid in a
liter of copying and record ink. As the quantity of free acid is cut
to a minimum in the formula, it is evident that the quality of the ink
w^ill suffer, if so much of the acid is neutralized. Writing ink contains
only half as much acid as the other, so the hard w^ater wall neutralize
almost 28 percent of the free mineral acid. Water that has been
softened by the zeolite, or base exchange, process is as alkaline as
before tliis treatment, and will neutralize as much of the acid.
To make a liter of one of the gallotannate inks for which formulas
are given, dissolve the gallic and tannic acids in about 600 ml of
water at about 50° C (122° F). For convenience, this can be done in
a 1 -liter measuring flask set in a vessel of warm water. As it may be
necessary to heat the water, the flask must not rest directly on the
bottom of the outer vessel, or it may be cracked by the heat. Stand
the flask on a flat coil of heavy ware or on some other device that will
keep it from actual contact wdth the heated bottom of the outer vessel.
Inks
13
Swirl the flask frequently to hasten the dissolving of the acids.
Then add the requisite amount of hydrochloric acid, and the crystals
of ferrous sulphate, which will soon dissolve. The flask and con¬
tents should then be allowed to cool to the temperature for which it is
calibrated, preferably 20° C (68° F). Meantime dissolve the dye in
a separate portion of 250 ml of water, and filter the solution directly
into the measuring flask. Rinse the vessel in which the dye was
dissolved with two or three small portions of water, not more than
100 ml in all, and pour each portion through the filter paper to wash
as much of the dye as possible into the flask. It may be noted in
passing that the dye should be so completely soluble that, after the
filter paper has been rinsed, there will be not more than a few solid
particles left on it. When the solution in the flask is at the correct
temperature, the carbolic acid can be added, and finally enough
water to make the total volume 1 liter. The ink must now be mixed
very thoroughly by inverting the stoppered flask a dozen or more
times.
Essentially the same procedure is followed when making the iron
gallate inks described in section II, 5(e).
tlydrochloric acid is a solution of a gas in water. The usual con¬
centrated acid contains about 36 percent by weight of the gas, the
rest being water. The Pharmacopoeia defines “dilute” hydrochloric
acid as containing not less than 9.5 nor more than 10.5 percent by
weight of the gas. The plain intent is to make 10-percent acid, and
directions for making it are given. There are reasons for suspecting
that some who make ink by one of the formulas given in this circular
do not understand how to prepare the dilute acid, and that they
regard the concentrated acid as 100-percent. If, acting on this
belief, they mix 10 parts by weight of the concentrated acid with 90
parts of water, they will have only 3.6-percent acid. The correct
way is to mix 100 parts by weight of the concentrated acid with 260
parts by weight of water. The 360 parts of the mixture will contain
36 parts of hydrochloric acid gas, or 10 percent.
The same principle must be applied if the strong acid is of some
other concentration than 36 percent. If it seems preferable to
measure the acid and water instead of weighing them, the density of
the acid must be taken into account. The density of the 36-percent
acid is about 1.19. If 100 ml is taken, it will weigh 119 g, and will
contain 42.8 g of hydrochloric acid gas, the last figure being 36 percent
of 119. To get 10-percent acid, add 309 ml of water, which wiU
weigh the same number of grams, for all practical purposes. This
will give a total weight of 428 g, containing 42.8 g of hydrochloric
acid gas.
Some manufacturers claim that sulphuric acid is better than hydro¬
chloric. Certainly it is easier to handle, because there are no dis¬
agreeable fumes. One part by weight of hydrochloric acid gas is
chemically equivalent to 1.345 parts of 100-percent sulphuric acid, or
to 1.416 parts (1.345/0.95) by weight of the usual concentrated acid of
95-percent strength (66° Baume; density 1.84). Hence, in the writing-
ink, the equivalent of 12.5 g of “dilute” hydrochloric acid containing
1.25 g of the gas is 1.25 X 1.416= 1.77 g of ordinary concentrated sul¬
phuric acid; or 1.77/1 .84 = 0.95 ml.
An ideal but quite impracticable way to keep iron inks is in glass
globes hermetically sealed by melting the glass together. A sample
14
Circular of the National Bureau of Standards
of about 200 ml preserved in this way had only a little sediment in it
after being kept for 9K years. Ink is sold in bottles, and these should
be nearly full, and the corks as perfect as possible, and not the inferior
ones with numerous cavities through which air can get in to the ink.
It is said to be best to keep the bottles in the dark.
In the appendix is a short discussion of dyes that can be used in iron
gallotannate and gallate inks.
7. AMMONIUM AMMONIUMOXYFERRIGALLATE INK
An ink that is of no present commercial importance, yet which gained
some prominence in the newspapers on account of the work described
in section 11,11, is a solution of ammonium ammoniumoxyferrigallate
in water. In 1908, two Rumanian chemists published the results of
their studies on iron-gall inks,® and among other things described the
method of preparing this compound, and suggested using a 7- to 8-per¬
cent solution of it as writing ink.
To prepare the compound, dissolve 7.5 g of ferric chloride (presum¬
ably FeCl3.6H20, though it is not so stated by the authors) and 7 g of
gallic acid in 100 ml of water. Add 15 ml of concentrated ammonia
water, and then 140 ml of strong ethyl alcohol. This precipitates the
ammonium ammoniumoxyferrigallate, which is filtered off and washed,
first with dilute alcohol and then with strong alcohol. The first wash¬
ing with dilute alcohol is intended to remove most of the ammonium
chloride formed in the reaction. This salt is nearly insoluble in strong
alcohol. At this Bureau, the first washing was done with a mixture of
12 volumes of strong alcohol and 10 volumes of water; a second wash¬
ing was with 20 volumes of alcohol mixed with 10 of water; while the
final washing was with strong alcohol alone. The black mass that
remains on the filter is dried in the air. It dissolves readily in cold
water, with an intense blue-violet color. When this solution dries
on paper it becomes insoluble in water in less than an hour, and black
in a few hours.
Unless some less wasteful method of preparation can be devised, it is
unlikely that the compound will ever be used for making commercial
ink. The liquid filtered from the original precipitate is intensely
black, and the wash-alcohol is strongly colored, so it is evident that
a considerable proportion of the material is lost. In addition, the
large volume of alcohol which must be used increases the cost. A few
experiments made at the Bureau indicated the possibility of preparing
the compound by dissolving ferric hydroxide in a solution of gallic
acid, and adding ammonia water to this solution of ferric gallate.
However, to make a large quantity of ferric hydroxide and to wash it
free from salts is a very difficult task.
8. AGING OF WRITING
The behavior of iron inks on paper is so important that it deserves
to be discussed in some detail.
The fresh writing is blue, except in the rare case of the ink containing
a black, instead of the usual blue, dye. In a few hours the writing
becomes perceptibly darker, because the ferrous salt in the ink has
* T. Silbermann and H. Ozorovitz, Bui. soc. stiinte Bucuresti 17, 43-67 (1908). The abstract in Chem.
Zentrbl. 1908, II, 1024, gives the method in sufficient detail. Mitchell and Hepworth (see p. 87-89 of the
book cited in footnote 1) also abstract the article, but have transposed the formulas for the ammonium salt
and the acid from which it is derived. In a footnote on p. 88, they refer to Zetsche’s criticism of the
work of Silbermann and Ozorovitz, but give the wrong volume and year of the journal in which the
criticism appeared. The correct citation Is Liebigs Ann. Ohem. 435, 233 (1924).
Inks
15
begun to be oxidized to black ferric gallotannate. Under ordinary
conditions of diffused daylight the writing should attain its greatest
intensity of color, a deep blue-black, in about a week. If the ink is
unusually acid, the color develops more slowly. On the other hand,
if the ink contains too httle acid, or if the acid is neutralized by expos¬
ing the fresh writing to the fumes of ammonia, the blackening will be
complete in a day or two.
The oxidation that causes the blackening does not cease abruptly
when aU the ferrous iron is converted into ferric iron, for the dye and
the gallic and tannic acids are also subject to oxidation. In the course
of time the dye will disappear. If this occurs before the two acids
have been affected to any great extent, the writing will still be black,
but no longer blue-black. This is normal for a well balanced ink. If
the ink maker depended more upon dye than upon iron gallotannate,
it does not seem impossible that the aging writing will never go through
the true black stage, but that when all the dye is gone, a substantial
part of the gallic and tannic acids will have gone with it, and then the
writing will have a brownish color. It is certain that if the paper
endures long enough, finally nothing will be left of the writing but
rusty lines of ferric oxide.
It is natural to ask how long it takes for all this to happen to the
writing, and the reader can draw his own conclusions after being told
of some of the factors which influence the rate at which the ink ages
on paper. First of all comes the ink itself. If it is truly a record ink,
the writing ought to last for centuries if it is kept under proper con¬
ditions. Ordinary writing ink can not be expected to last as long as
record ink, and if both are used in the same document, as when two
persons sign it, each with his own fountain pen, one might draw the
conclusion 20 or 30 years later, that one signature is much more
recent than the other. If one of the inks was too dilute, and also
contained too small a proportion of gallic and tannic acids for the
amount of iron, the VTiting might become rusty in a comparatively
few years. The fiuidity of the ink and the absorptiveness of the
paper influence the quantity of ink in the written characters, and
thus play their part in the aging. If the writing was blotted instead
of being allowed to dry naturally, the ink is handicapped at the start.
An inspection of old notebooks, ledgers, and similar records is apt to
disclose considerable differences in the appearance of writing of the
same date and in the same hand. Such differences are outstanding
in the notebooks and lecture notes discussed in section II, 9.
The appearance of the writing after a number of years have gone
by will also depend upon how it has been kept. If it has been much
exposed to light and has been in a damp place, the writing will look
older than if it has been kept in a dry atmosphere and in the dark.
Apparently the character of the paper is not to be disregarded.
According to Schluttig and Neumann, fresh writing will darken much
more rapidly on some kinds of paper than on others. It is not to be
supposed that this hastening of the oxidation by the paper will cease
in a short time. It seems more reasonable to think that the paper
will continue to affect the ink over a period of years. If it does,
then of two pieces of writing, identical in every respect except the
kind of paper, one will age more quickly than the other.
101251°— 36 - 3
16
Circular of the National Bureau of Standards
9. DATING A DOCUMENT
Examiners of “questioned documents’’ base tlieir conclusions as to
the age of the writing to a considerable extent on its appearance. As
explained in the preceding section, writing done with iron gallotannate
ink of the modern type in the course of time undergoes an orderly
series of changes of color. Usually blue at first, it begins to blacken
in a short time and reaches its greatest depth of color, a blue-black,
in the course of 2 weeks, more or less. The writing continues to be
acted upon by the oxygen of the air, so that first the dye, and then the
gallic and tannic acid compounds, are oxidized away. Finally ferric
oxide — rust — is all that is left. How long it takes for all these changes
depends on a number of factors, as explained in the preceding section.
In examining a document, the first step is to find out what kind of
ink was used, by applying small drops of appropriate chemical
reagents to selected parts of the individual letters. If it was an iron
ink, the presence of blue dye is tested for. The ease with wUich cer¬
tain reagents dissolve the dried ink is also determined. The results
of these and other tests, together with whatever collateral evidence
can be gathered, are depended upon by those who testify as to the
age of a document, or the relative ages of two or more signatures,
supposedly written at the same time, yet suspected of having been
written at different times. Because of the many factors that influence
the rate at which writing ages, the National Bureau of Standards has
always declined to express any opinion, based upon tests of the ink,
as to the age of any sample of writing. However, the Bureau was
at one time practically forced to examine a number of samples of
writing in order to get some idea of the length of time it takes for the
blue dye to disappear. In connection with a case in court, a chemist
of the Bureau tested the writing of three letters of disputed age, solely
to find out what kind of ink had been used, whether iron gallotannate
or a solution of a black dye. It turned out to be an iron ink, and in
reporting this it was added that the blue dye could be detected.
This wa.s at once seized upon by two experts, wFo said that the pres¬
ence of the dye proved that the letters could not be more than 15
years old, which was an important point at issue. That the dye will
disappear within this time seems to be the general opinion, though
some few of those who make a study of such matters think differently.
The belief that all the dye will disappear within a limited number of
years is expressed by Mitchell and Hepworth ® in discussing the
results of tests made by them on a number of bank checks of different
ages. They say: “In abnormal cases, wUere an excessive amount of
ink had been used, some diffusion of the blue pigment may occur even
after the lapse of twelve years.”
vStatements such as these led to the examination of a large number
of samples of writing, 116 in all, in laboratory notebooks at the
National Bureau of Standards, in university lecture notes, and in legal
papers.^® The writing was variously dated from 1851 to 1918. The
tests were made in 1933, so all the writing was 15 or more years old.
It can not be said that the writing was abnormal, nor that an excessive
amount of ink had been used. Indeed, because on a given page the
writing varied from a deep black to a pale color, care was taken to
® C. A. Mitchell and T. C. Hepworth, Inks, Their Composition and Manufacture, 3d ed. (Chas.
GrifDn & Co. (Ltd.), London, 1924), p. 182.
What follows in this and the next 3 or 4 paragraphs is a summary of an article by C. E. Waters, Blue
dye as evidence of the of writing, Ind. Eng. (Jhem. 25, 1034 (1933).
Inks
17
test a medium-dark character as well as an intensely black one on each
page. So in nearly half of the tests, the writing contained less than a
normal amount of ink. Although the writing of the lecture notes
(1895-96) and in the notebooks (1904-18) was all in the same hand,
many different lots of ink must have been used. Some had evidently
been blotted, and some allowed to dry naturally; some was done with
a moderately blunt pen, and some with a fine-pointed one. It can
be fairly said that these samples can be regarded as writing picked at
random, and that the results obtained with them can validly refute
the statement that the blue dye disappears within 15 years.
The tests were made as follows: A small drop of distilled water was
placed upon a selected stroke of a letter or niiineral. After it had
stood for 10 seconds, it was removed by pressing down upon it, with
a finger tip, a small piece of white filter paper with a fairly smooth
surface. After 10 seconds, the paper wa.s removed and examined for
signs of blue dye. In many of the 116 tests, the result v/as recorded
as ‘‘none”, and in a few as “doubtful.” When there was blue dye on
the filter paper, the amount was estimated on an arbitrary scale in
four steps, “trace”, “faint”, “distinct”, and “strong.”
Of two tests made on writing dated 1881, one showed dye faintly
and the other distinctly. Two tests of writing of 1879 were negative,
while one test each, representing 1851, 1852, 1865, 1872, and 1874,
gave stains varying from faintly to strongly rusty, with no trace of
blue. Writing of 1883 gave a negative test, but that of 1886 still
contained blue dye. Of 12 samples dated 1895 and 1896, all gave
positive tests. It turned out that the dates 1901 to 1918, inclusive,
had little or no connection with the results. If the testing had been
limited to writing that was just 15 years old— 1918 — what the two
experts said would have been upheld, because four samples of that
year failed to show any sign of dye. Yet writing dated from 1901 to
1917 was found to stain the filter paper to all degrees of intensity, at
random. Of the 116 tests, 60 gave positive evidence, though slight
in many cases, while 56 were negative or doubtful. This and the
scattering of the positive and negative tests over the years show con¬
clusively that the presence or absence of dye is no dependable criterion
of the age of vniting done within 50 years.
The method used by Mitchell and Hepworth differed from that of
the National Bureau of Standards. They treated the writing with a
5-percent solution of oxalic acid to dissolve the surface film of iron
salt w^hich, according to their belief, will not let the dye escape from
writing that is several years old. Oxalic acid removes this film and
lets the dye dissolve and diffuse into the drop of liquid. Because of
the many positive results obtained by the filter-paper method, 31
samples of the writing already examined were tested by the procedure
of Mitchell and Hepworth, but with distilled water instead of a solu¬
tion of oxalic acid. A tiny drop of water was allowed to stand on the
selected letter or numeral. In most cases a deep black one was
chosen. As before, the writing of 1881 gave positive results when
tested in two places, because the two drops of water were faintly
colored blue. Four tests of writing dated 1886 were rated from trace
to strong. The writing of 1895 showed no dye in six tests, while for
1896, four out of eight tests were negative. Two samples dated 1910
gave strong evidence of diffusion of dye within 25 seconds.
18
Circular oj the Natioual Bureau oj Standards
It is evident that more than half of these results fail to support the
claim that all the dye disappears within 15 years. If so mild a reagent
as distilled water could bring out such differences as those described in
this section, one should be cautious about drawing inferences from the
action of other chemicals upon v/riting of unknown age. It is not even
safe to be too positive about the age of writing that has a rusty
appearance, with little or no black iron gallotannate. The ink may
have been so poorly balanced in composition, or the writing may have
been kept under such unfavorable conditions as to give a false idea of
the age of the document. Like other matters, any expression of
opinion relating to the age of writing lends itself admirably to forensic
disputation.
During the past few years, a new method for ascertaining the age of
writing has received notice in technical and other journals. This is
the so-called “chloride test,’’ which is based upon the fact that nearly
all writing ink contains hydrochloric acid or metal chlorides. The
chlorides spread gradually from the dried ink marks, supposedly by
means of the film of moisture on the fibers of the paper. The extent
of the spreading is taken as a measure of the age of the writing. It will
be clear to the reader that the rate at which the chlorides spread from
the strokes of the writing will depend upon a number of factors, for
instance, the humidity of the air, which influences the film of moisture,
and the character of the paper. In making the test,^^ the writing is
treated with dilute nitric acid, containing silver nitrate to convert the
chlorides into silver chloride, which is relatively insoluble and remains
on the paper where it is formed. The writing is also decolorized by
treating it with a solution of a nitrite or of permanganate. After
the excess of silver nitrate is washed out of the paper with dilute
nitric acid, the silver chloride is reduced to silver by treatment with
an alkaline solution of formaldehyde or with sodium hydrosulphite.
Silver obtained in this way is so finely divided that it looks black, so
there is form.ed by the last reaction a zone that is darkest next to the
strokes of the writing, and gradually becomes paler until it blends with
the color of the paper. Because paper contains chlorides, acquired in
the process of manufacture or from handling, it will be more or less
darkened all over when the chloride test is applied.
According to Cornish, Finn, and McLaughlin, the chloride test is
unreliable. The paper in which their work is described ends with a
list of experts or manufacturers who “were unanimous that such a
test could not determine the age of ink writing under normal condi¬
tions, and that there was no reliable method Imown to science whereby
the age of ink writings on documents could be accurately determined.”
The authors cited in footnote 11 also discuss the spreading of sul¬
phates from the strokes of the writing into the paper, as another means
of determining the age of the writing.
An account of some tests made with a bleaching agent (hydrogen
peroxide) upon 7 samples of writing, 1 day, 6 months, and 1, 2, 6, 14,
and 22 years old, was found in an unexpected place. The writer
said that in order to determine the age of writing by the rate at
which it is attacked by bleaching agents, certain “precautions” must
be observed:
” O. Mezger, H. Rail, and W. Heess, Z. angew. Chem. 44, 645 (1931).
>2 R. E. Cornish, J. Finn, Jr. and W. McLaughlin, Age of inks by the chloride test. Ind. Eng. Chem. News
Ed. n, 316 (1934).
13 Workshop Receipts 5, 88 (Spon «fe Chamberlain, New York, 1903). The article, Jnl:, is signed by R.
Irvine.
Inks 19
1. The inks must be those known as ordinary writing inks, prepared from
iron and chromium salts and galls.
2. Writing dried by means of blotting paper is naturally more easily removed
than writing which is allowed to dry on the surface of the paper; and light writing
is somewhat more easily removed than coarse and heavy writing.
3. The bleaching solution must be exceedingly dilute, otherwise the action is
so rapid and powerful that both old and new writings are removed almost
simultaneously.
4. The action must be carefully watched, so as not to be too long continued.
Lastly, very old writing which has become brown by age, although it resists the
action of weak solutions of bleaching powder and hydrogen peroxide, will show
signs of giving way almost instantly when acted upon by dilute nitric, hydro¬
chloric, and oxalic acids.
To put ‘‘precaution’^ 2 in other words, unless all the writing was
exactly the same at the start, tests made upon it will not determine the
relative ages of different parts of the writing. It must be evident
that questioned documents must be taken as they come, whether or
not the writing was blotted, and in spite of some of it being light, and
some coarse.
10. RESTORATION OF FADED WRITING
The choice of iron gallotannate ink for records is generally based
upon its long resistance to fading, when kept under proper conditions,
and little thought seems to be given to another advantage it has over
an ink that is only a solution of a dye. Wlien a dye fades, it some¬
times leaves literally nothing behind on the paper, and if traces of
oxidation products remain, they can not be detected readily, if at all.
When an iron ink fades, it leaves small quantities of iron oxide in the
lines of the letters, and this makes it possible to restore the legibility
of the writing.
The vapors of ammonium sulphide will change the ferric oxide into
ferrous sulphide, mixed with “molecular” sulphur. The ferrous
sulphide will be brown or black, according to the amount of it.
Writing restored to legibility in this way does not last long, because
the iron sulphide is oxidized to sulphate in a few days, and there
may be bad aftereffects upon the paper.
A 2- or 3-percent solution of tannic acid will also blacken the ferric
oxide. This is by far the best treatment, because it produces essen¬
tially the same black compound that was in the writing before it
faded, and because neither a strong acid nor a metal salt is left in the
paper. In addition, it has been found (see section II, 11) that tannic
acid does not cause deterioration of paper, in an accelerated aging
test.
Again, a slightly acidified solution of potassium ferrocyanide will
change the ferric oxide into prussian blue. This is a very permanent
color, but the salts left in the paper by the treatment may later cause
trouble.
The tannic acid and the ferrocyanide solutions can be applied by
means of a brush. Another way is in a letterpress, by placing cloths
or pieces of white blotting paper moistened with the solution in con¬
tact with the faded writing, and keeping the whole under pressure for
a few minutes. A disadvantage of any chemical treatment is that
iron is found in practically everything. Paper and dust contain it,
and if the document has been much handled, it will have a surface
coating, containing iron, that comes from dirty hands. The chemicals
that are applied are not selective in their action, so that the writing
20 Circular oj the National Bureau of Standards
is restored as dark lines upon a less dark and unevenly colored back¬
ground.
If a source of ultraviolet radiation is at hand, it is better to defer
the application of cliemicals until the effect of this radiation has been
tried. Under the right conditions the iron oxide will glow with a
phosphorescent light that can be photographed. It is thus possible
to get an exact copy of the writing without running the risk of damag¬
ing the document permanently.
A number of years ago, when it was decided to place the Declara¬
tion of Independence on exhibition in the Libraix" of Congress, the
National Bureau of Standards was consulted about the ad^dsability
of intensifying the vudting by chemical treatment. After serious con¬
sideration of the uncertainty of full success, and of the danger of the
action of the chemicals upon the document, it was decided that it
would be better not to tamper with it, at the risk of ruining it forever.
11. EFFECT OF WRITING INK UPON PAPER
As was said on an earlier page, the condition of old documents in
European libraries shoves that not all the inks with which they were
written were balanced in chemical composition. According to what
we read, some of the documents are in excellent condition, while others
have suffered to a greater or less extent from the action of the ink
upon the paper. In some cases, only the unwritten margins of the
pages remain, for all the rest of the paper has fallen to pieces. It is
generally believed that this destruction of the paper has been caused
by an excess of sulphuric acid in the ink. It is v^ell established that this
acid makes paper brittle. The excess of acid may have come from
carelessly made ferrous sulphate. Another explanation of the destruc¬
tion of the paper is that it has been brought about by the iron oxide
in the ink. This oxide is supposed to act as an ‘hxygen carrier” to
cause the weakening of the paper, just as a rusty nail, which is coated
with iron oxide, attacks wood.
The effects of 12 vniting inks, of different kinds, upon 7 kinds of
writing paper were studied by the Bureau. In addition, solutions of
gallic, tannic, and hydrochloric acids and of ferrous sulphate, in vari¬
ous combinations, but in the same concentrations as in writing mk,
were tested with one of the papers. The tests were made by draving
parallel lines of a definite vddth and equally spaced across one-half
of each sheet of paper. The folding endurance of the uninked and
the inked parts of the paper was determined before and after subject¬
ing the paper to an accelerated aging test. This test consists in
keeping the paper at 100° C (212° F) for 72 hours, and measuiing
any changes in the folding endurance, which gives the best indication
of am^ weakening of the paper.
The details of the investigation must be obtained from the original
article. It vill suffice to say here that all the inks tested increased
the deterioration of the paper in the accelerated test. The least harm¬
ful ink was a solution of 20 g of ammonium ammoniumoxyferrigallate
in 1 liter of water. In the aging test one of the papers, without ink,
retained 72 percent of its original folding endurance. The same
paper, with lines dravm with this ink, retained 68 percent of its folding
E. W. Zimmerman, C. G. Weber, and A. E. Kimberly, Relation of ink to the preservation of written
records. J. Research NBS 14, 463-468 (1935) RP779. A summary of this work is given by B. W. Scribner
and A. E. Kimberly, on p. 24 of the Bureau's Miscellaneous Publication M144, Summary Report of Bureau
of Standards Research on Preservation of Records. Both publications give numerous references.
Inks
21
enduraDce, in the aging test. This difference is practically negli¬
gible. The next best ink caused a drop to 58 percent of the original
folding endurance. This second ink was made by dissolving 15 g of
dialyzed prussian blue, without the aid of oxalic acid, in 1 liter of
distilled water. A commercial “acid-proof” ink made by dissolving
Prussian blue with oxalic acid would certainly cause greater deteriora¬
tion of paper than the ink made by the Bureau. Solutions con¬
taining tannic and gallic acids, or these two acids with hydrochloric,
had no harmful effect upon the paper. Ferrous sulphate, either by
itself or mixed with the acids, caused weakening of the paper. Hy¬
drochloric acid is so volatile that it escapes from the paper before it
can do any harm. It was not necessary to make a test with sulphuric
acid, because it has long been estabhshed that it is extremely harmful
to paper.
12. INK ERADICATORS
When writing is removed wdth a steel eraser or v/ith one made of
rubber containing an abrasive material, the surface of the paper
suffers, and it is not easy to write again over the same spot. To avoid
this, various ink eradicators, chemical solutions that dissolve blue-
black ink from paper, have been devised. Wliether they will also
dissolve dye inks, which are taken up in the pages which follow, de¬
pends upon the chemical nature of both the dye and the eradicator.
If the latter has a strong bleaching — usually oxidizing— action, it is
more likely to remove dye inks than if it acts merely as a solvent.
A drawback to the use of chemicals to erase writing is their effect
upon paper. At first, no harm appears, but some of the chemicals
will always be left beliind, and finally damage the paper. Usually a
brownish spot appears where the eradicator was applied, and after
a while the paper is found to be brittle. The damage may be lessened
hy wasliing the spot. YlTien the erasure is complete, dry the spot
with a blotter, and then appty one or two drops of pure v.^ater. After
a few moments remove the water with a blotter, and repeat the opera¬
tion once or twice. This will not completely remove the chemicals,
but it will help to save the paper.
The usual two-solution eradicators consist of a solution of bleaching
powder or of javelle water and a dilute acid. Either of the first two
has a bleaching action, which is made more rapid by the action of
acids.
Cartons of bleaching powder, or “chloride of lime”, can be bought
in drugstores. Javelle water is made by adding sodium carbonate
(sal soda, washing soda) to a solution of chloride of lime. Because
the directions for making it are always printed on the carton, they
need not be given here. If the solution of bleaching powder is to be
used for removing ink, it should be diluted with water, to the same
total volume as when javelle water is made from it by adding a solu¬
tion of soda. Either solution will bleach, but the action is more rapid
if an acid is used with it. A 5-percent solution of acetic acid can be
used. Strong vinegar contains about this percentage of the acid,
but if any great quantity is needed, it will be cheaper to buy the
commercial 28-percent acid. One volume of this with 4.5 volumes of
water will make a mixture that contains almost exactly 5 percent of
acetic acid.
Oxalic acid, a poisonous crystalline substance, will dissolve dried
iron gallotannate ink. The small packages of “straw hat cleaner”
22
Circular of the National Bureau of Standards
sold in drugstores are apt to be of this acid. It should be dissolved
in about 20 times its weight of water. Much safer is a mixture of
equal weights of tartaric and citric acids, dissolved in about the same
amount of water as for oxalic acid. Citric acid is what makes lemons
sour, and lemon juice has long been employed in the home for remov¬
ing iron rust from garments.
In Farmers' Bulletin 1474, Stain Removal from Fabrics: Home
Methods, the Department of Agriculture tells how to remove the
inevitable ink spots from clothing and other fabrics.
III. OTHER KINDS OF INK
1. CARBON INKS
(a) CARBON WRITING AND DRAWING INKS
Carbon cannot be bleached by any amount of exposure to intense
light, and it resists attack by chemicals that will quickly destroy
paper. If carbon could be dissolved in water, it would be ideal
material for making black writing ink. India ink, which has abeady
been mentioned briefly, is not a solution of carbon, but a suspension
of it in water containing gum or glue. To those who write with a
brush, it matters little if the carbon settles to the bottom of the
saucer of ink, for it can be stirred up with each dip of the brush.
Fortunately for those of us who must use carbon ink vdth a pen, we
have learned how to keep the carbon from settling to the bottom.
If some purified lampblack is stirred with water and then left to
itself, after a time it will settle and leave clear water above. If the
Diixture is ground a long time, and then allowed to stand undisturbed,
the carbon will not settle so quickly as before, and part of it may be
very slow, indeed, in reaching the bottom. If instead of pure water, a
solution of some plant gum, of shellac and borax, or of soap is used,
after thorough grinding the carbon will tend to remain in suspension
a long time. The difficulty in preparing a permanent suspension of
carbon lies in the grinding. If it is ideally complete, no two particles
of carbon will touch one another, much less cling together, but each
will be separate from all the others, and each will be coated with an
adsorbed film of gum, shellac, or soap. The carbon will then have
almost no tendency to settle. Microscopic examination shows that
ordinary lampblack consists of clusters of extremely small particles.
Grinding does not make these ultimate particles smaller, but separates
them from one another.
Carbon black can now be obtained that disperses readily when
stirred with water. It comes in the form of a stiff paste, which is
probably made by grinding carbon black with a solution of gum or
some other colloidal material.
The manufacturers of black drawing ink are very successful in
making practically permanent suspensions of carbon. According to
one manufacturer, the ink mixture is ground for 3 or 4 weeks m a
ball mill. If this is necessary, it is a sufficient reason why carbon ink
can not be made satisfactorily by hand grinding.
Water that is clouded by extremely fine particles of clay in suspen¬
sion will clear quickly if some salt is dissolved in it. Similarly, the
carbon in an ink will settle rapidly if some acid is added. Alkali, on
the other hand, makes the suspension of carbon more stable. The
ammonia that can be smelt in some drawing ink is a mild alkali.
Inks
23
Because of the sensitiveness of carbon suspensions to acids, carbon
inks can not be mixed with iron gallotannate ink, and a fountain pen
that has held the latter must be cleaned with extreme care before
filling it with a carbon ink. Carbon inks might be popular if it v/ere
not so easy to ruin them by ignorance of their peculiarities, or by
carelessness.
(b) PRINTING, CANCELING, AND OTHER CARBON INKS
Black drawing ink contains only a small percentage of solid matter,
and it does not differ greatly from clear inks in fluidity and working
qualities. Other kinds of carbon inks range in consistency from only
slightly viscous fluids to stiff pastes, which may contain as much as
25 percent by weight of carbon. The carbon in a canceling ink
should be carried into the paper, and remain there in spite of attempts
to remove the marks by washing. Mimeograph and other duplicat¬
ing-machine inks require great care in their formulation and manu¬
facture. If the mimeograph ink is not made just right, the copies
made with it may be too pale, because the ca.rbon clogs the stencil,
instead of passing through it readily. Again, the ink may go through
the stencil too freely, and may become smeared over the face of the
stencil, and from that to the paper.
Printing inks contain more carbon than any other kind of ink, and
among themselves they differ widely. Some are thick liquids, and
others stiff pastes, with all consistencies between. It is necessary
to adapt the physical properties of the ink to the kind of printing to
be done. The same ink can not give equally good results in printing
from ordinary type, from a lithographic stone, a halftone cut, and an
engraved plate; and the paper introduces another important factor
in the results.
Formulas for various kinds of carbon inks are to be found in books,
but they should be regarded as only suggestions. The character of
the finished ink depends upon the physical and chemical properties of
the ingredients, upon the amount of each that is used, and upon how
the ink is made. The various commercial forms of carbon that are
used as pigments differ among themselves in physical properties, and
can not be used indiscriminately. There are no Federal specifications
for any of these inks, because there are no laborator}^ tests that can
take the place of actual trials on the press, and with the paper and
the kind of work for which the ink has been made. The ink manu¬
facturer has his working formulas, but he would not turn over the
actual production to an unskilled person.
It is possible to measure some of the properties of some of the
ingredients of a printing ink, but there is no way by which to predict
exactly what the finished ink will be like. The consistency, for
instance, depends to a great degree upon what is called the oil absorp¬
tion of the pigments. This differs according to their chemical com¬
position, and is closely tied up with the fineness and degree of dis¬
persion of the pigment. The last is largely dependent upon the
grinding.
The difficulties involved in the manufacture of a pigment ink are
well illustrated in the description of the process by which the United
States Government Printing Office makes mimeograph ink.^^
If Government Printing Office Tech. Bui. 16, Standard Mimeograph Ink and Paper.
101251 '■—30 - 4
24
Circular of the National Bureau of Standards
2. DYE INKS FOR WRITING
As explained on an earlier page, when an iron ink fades with age,
it leaves behind on the paper at least a little iron oxide, and thus it
is possible to restore the v/riting to legibility by suitable chemical
means. When the ink is merely a solution of a dye, there is no possi¬
bility of such a restoration when the writing fades. Oxidation of the
dye forms volatile products which escape into the air, or maybe
small amounts of other products which remain in the paper, but
with which there is no certain, dependable way of forming colored
compounds. For these reasons dye solutions are not regarded as
suitable for record inks. On the other hand, they have advantages
over iron inks. They keep almost indefinitely in the bottle, are
seldom corrosive, and because they contain less solid matter than
iron inks they do not form thick deposits if they dry on pen points.
Besides, if a crust does form, from a d^e ink, on an unwiped pen, it
will usually redissolve when the pen is again dipped into the ink,
something that does not happen to the incrustation from an iron
ink. A liter of the standard writing ink of the Federal specification
contains 35 g of nonvolatile solids, while the same volume of a dye
ink may contain no more than 10 g of solids, and sometimes a great
deal less. In Federal Specification TT-I-549, for red ink, the
standard is made by dissolving only 5.5 g of crocein scarlet SB (C.I.
252; Sch. 227) in a volume of 1 liter. A still weaker solution of
methyl violet B (C.I. 680; Sch. 515) would sufiice for that color.
Although dye inks are not considered suitable for records, they
are not to be condemned on that account. They are excellent for
ordinary correspondence, and for writings that are not meant to be
records of permanent value. If kept in a dry place, and away from
the light, there is no reason why writing with dye ink should not
last for many decades. In some of the oldest record books of the
Post Office Department, dated before 1800, there is writing with red
ink. It may have been bright red at first, but it is now brownish
red, and of surprisingly good intensity. At the National Bureau of
Standards is a book in which there are several press copies of letters
written early in 1901. The inks represented are iron gallotannate,
blue from a ‘^copy blue” typewriter ribbon, violet from printing ink
containing dye of that color, and another violet from rubber-stamp
ink. There are also two red lines on a page with a rough drawing.
Evidently the drawing was made on a scratch-pad, and when the
sheet was torn off in order to make the press copy, a little of the red
glue (“padding compound”) that was on two edges of the pad, came
with it. This was enough to make the two red lines in the press copy.
A press copy contains only part of the coloring matter of the original,
and it might be expected to fade quickly on that account. In spite
of this, the copies are still of good color, 35 years after they were
made, and there is nothing to suggest that they will not last as long
again, if they are kept under the same favorable conditions as here¬
tofore.
There are no Federal specifications for writing inks made of dyes
of other colors than red. There are many water-soluble dyes, and
it is possible to make inks of almost any shade and hue by dissolving-
suitable dyes in water. If it should turn out that a particular ink
has a tendency to “feather”, or make blurred spreading lines on
paper, this can be prevented by dissolving in the ink some gum
Inks 25
arabic, say 20 or 30 g in a liter. Another expedient will be found
in the account of recording inks.
Many dj^es have an antiseptic action, so their solutions do not
become moldy, though no preservative is added to them. With
other dyes it is necessary to use about 1 g of phenol or other preserva¬
tive in a liter of ink. If a dye is just on the border line, the addition
of gum arabic to the ink might encourage the growth of mold that
would otherwise not thrive. No systematic work has been done on
this subject by the Bureau. Anybody who is interested in a par¬
ticular d3^e or dyes can easily test them for himself by inoculating the
solution with mold spores and keeping it in a dark, warm place for
about 2 weeks. It is sometimes noticed that mold will not grow on
ink, but will do so on the cork of the bottle in which the ink is kept.
When a dye ink is used in a fountain pen, sometimes trouble is
experienced by incrustations of dried dye on the point of the pen.
This can be avoided by mixing with the ink about one-tenth its vol¬
ume of glycerin, which retains enough moisture to keep the dye from
separating in solid form. If much more glycerin is added to the ink,
it will dry too slowly on paper, and for that reason the writing easily
becomes smudged. Indeed, even if only one-tenth volume of glyc¬
erin is added, the first few words written with a fountain pen that
has not been used for a few days may show by their slow drying that
evaporation has caused a concentration of the glycerin at the pen
point.
(a) WASHABLE INKS
Now and then somebody asks how to make washable ink. In the
generally accepted sense, a v/ashable ink is one that can not be
removed by washing; in other words, it is indelible ink. The inquir¬
ers, however, use the word in exactly the opposite sense, for they
want to make ink that can be washed out of fabrics easily and com¬
pletely. No study has been made of this subject by the Bureau,
but in theory, at least, it is easy to make such inks by selecting dyes
that do not fix themselves upon the fabric except with the aid of a
mordant. Mordants are substances that form insoluble compounds
with the dyes, so they are much used in dyeing, in order to fix the
colors more lastingly to the fabrics. If the fabric is made of cotton
or other vegetable fiber, the direct, or substantive, dyes should be
avoided for washable inks, because, as their name implies, they are
taken up directly by the fabric without the aid of a mordant. If the
fabric is silk or wool the problem is far more difficult, because these
libers can be colored by dyes of almost any class.
(b) QUICK-DRYING INKS
Another question that is sometimes asked is how to make writing
ink dry more rapidly. Ink dries partly by evaporation and partly
by soaking into the paper. Alcohol evaporates more rapidly than
water, and also is absorbed more rapidly by paper. Good writing
paper is sized with glue or rosin, to keep watery solutions that are
either neutral or slightly acid from being absorbed so rapidly that
they will spread too widely. In other words, the paper is sized in a
way intended to keep ordinary writing inks from making blurred
marks. A strongly alkaline ink would quickly show on the opposite
side of the paper. The situation will be even worse if a large propor¬
tion of alcohol is added to the ink, which will then penetrate paper
26
Circular of the National Bureau of Standards
and even thin cards almost in an instant. The proportion of alcohol
that can be added safely to a given volume of ink will depend upon
the kind of ink and upon the paper as well. The results of a few
experiments may be of interest. Starting with 10 volumes of ink, a
m-easured volume of alcohol was added to it, and the mixture was
then tested by writing and by drawing hght, medium, and heavy lines
with an ordinary pen upon scratch-pad paper and the bond paper
used for Government correspondence. Another measured volume of
alcohol was then added and the mixture tested as before. The addi¬
tion of alcohol w'as continued until the lines were strongly feathered.
With 10 volumes of the standard blue-black wTiting ink and 3.5
volumes of alcohol, there was slight feathering on the inferior paper,
as there was on the bond paper when the alcohol was increased to 4
volumes. With the standard red writing ink, feathering was produced
on the pad paper by 3 volumes of alcohol and on the bond paper by
4 volumies.
Acetone is considerably more volatile than alcohol, so its effect upon
red ink was tried. With scratch-pad paper there was miore feathering
than with alcohol, but on the bond paper there was none when 11
volumes of acetone was mixed with 10 of the ink.
In every case, beginning wdth the unmixed inks, the medium and
heavy lines showed a spreading of the ink beyond the limits of the two
parallel scratches made by the pen. Also, when the lines were feath¬
ered, some penetration of the ink to the reverse side of the paper
occurred. As the proportion of alcohol or acetone to ink was increased,
there v/as a distinct tendency for the lines to become broader. How¬
ever, a line w^as not considered as being feathered unless its edges were
noticeably uneven.
3. PRUSSIAN BLUE INKS
Prussian blue is not a dye, but it has as great coloring powder as
some dyes. It is ordinarily quite insoluble in water, but a kind
known as soluble prussian blue can be prepared. It does not form a
true solution, as salt, sugar, and many other substances do. It is
more like the suspensions of carbon and clay that have been men¬
tioned, but the particles of blue are so small that the suspension, or
‘‘coUoidal solution'', looks perfectly clear. The blue is first formed
as an insoluble precipitate, which must be washed until all the salts
it contains are removed. It is then soluble in water.
The salts can be removed most easily by dialysis, because the pre¬
cipitated blue soon so clogs an ordinary fiter that water will barely
pass through a comparatively thin la3^er of the pasty precipitate. To
wash as little as 10 g of prussian blue on a filter take 3 or 4 w-eeks,,
and because it runs through the paper towards the end of the washing
much of it may be lost. Prussian blue is formed when ferric salts and
a ferrocyanide, both dissolved in water, are mixed. It is said that
there should be an excess of ferrocyanide in order to get a good soluble
blue. Perhaps the principal reaction is as follows:
FeClg.eHsO + K4 (Fe (CN) 5) = FeK (Fe (CN) 9) + 3KC1 + 6H2O.
270.3 422.3 306.8
From the molecular weights given below the equation the weights
of ferric chloride and of potassium ferrocyanide needed to make a
desired v/eight of prussian blue can be calculated, but only approxi-
Inks
27
mately, for there are unavoidable losses, as well as at least two other
reactions. Suppose it is desired to make 30 g of the blue, which is
about as large a quantity as can be handled readily, there will be
needed 27 g of ferric chloride. This is dissolved in 500 ml of water,
and the solution is cleared by adding a few drops of hj^drochloric acid.
The solution should not be brown and slightly cloudy, but of a clear,
bright yellow. The weight of potassium ferrocyanide will be 42.2 g,
plus an excess of, say, 10 percent, or 46 to 47 g in all. This is dis¬
solved in 500 ml of water in a 1 -liter beaker. The iron solution is then
poured, a little at a time and with vigorous stirring, into the solution
of ferrocyanide. The precipitate is allowed to settle overnight, the
layer of clear solution above the precipitate is removed, preferably by
suction or siphoning, and the beaker is filled by adding distilled water.
After the precipitate has been stirred, it is again left to settle. ¥7hen
the blue has been washed a few times in this way, it will no longer
settle ’well. The contents of the beaker are then poured into a large
funnel with a close-fitting filter paper, and the water is allowed to
drain off as completely as it will. The pasty mass is now transferred
to a bag made by wetting a large square of ordinary, not waterproof.
Cellophane, and gathering and tying the edges around the stem of a
funnel. Most of the transferring can be done by scraping the pasty
mass from the filter and placing it upon the middle of the wet sheet
of Cellophane, before it is tied around the stem of the funnel. What
adheres to the filter is transferred ’with the aid of a jet of water from
a wash bottle. When this is accomplished, the funnel is removed
from the bag, and the latter is hung in a 4-liter beaker nearly filled
with distilled water. The water should come not quite up to the level
of the mixture in the bag. Although the excess of ferrocyanide and
the potassium chloride formed in the reaction pass out into the dis¬
tilled water, water also passes into the bag and may fill it if it is
im.mersed too deeply in the water. For this reason the bag must be
made of a piece of Cellophane at least 20 inches square. The water in
the beaker must be renewed at least once a day, for at least a week or
10 days. Wdien it is believed that the salts have been washed out,
take a drop of the liquid out of the bag and let it dry in the air. Then
put a drop of distilled water on the dried residue. If the prussian blue
has been washed enough, it will at once dissolve in the water. Other¬
wise, the dialysis must be continued for a few more days.
The thoroughly washed prussian blue will run through any filter,
so the only way to get the dry substance is to hang the Cellophane
bag and its contents in a warm place for the water to evaporate.
WTien the mass dries to a stiff paste, the top of the bag can be cut
off and discarded. It is generally discolored with ferrocyanide.
Then spread the bag open, so the paste will dry more rapidly. At¬
tempts to hasten the drying by heating, even to the temperature of
boiling water, is apt to cause much of the blue to become insoluble.^®
Solutions made by dissolving 5, 10, and 15 g of prussian blue,
prepared in the way just described, in a liter of distilled water, when
kept for nearly 4 years deposited not more than traces of sediment.
During this time the solutions were kept in the dark, or in dim light,
though this probably had no influence on the result. When a 0.5-
percent solution, in a corked bottle, was exposed to summer sunlight
'9 It may be noted in passing that long-continued dialysis failed to make cupric ferrocyanide soluble,
and that a suspension of the brown compound in water was not made clear by the addition of oxalio
acid.
28
Circular oj the National Bureau of Standai^ds
at a south window for 56 days, there was no precipitation of the
Prussian blue. Other solutions of the same lot of blue v/ere made
with different small amounts of oxalic acid, because this is usually
a component of prussian blue writing inks. These solutions kept
no better than those made without oxalic acid.
Various published formulas for prussian blue ink require amounts
of oxalic acid equal to one-fourth, or even one-half of the weight
of the dry blue. The best results are obtained if the mixture of
blue and acid is just covered with water and allowed to stand for
several hours before adding the full amount of water called for.
The peculiar action of oxahc acid upon prussian blue can be taken
advantage of for making really soluble blue, without starting with
ferric chloride and potassium ferrocyanide. Commercial prussian
blue, for instance the finely powdered form sold as a paint pigment,
is mixed with one-fourth its weight of oxalic acid and enough water
to make a thin paste, and after about 24 hours is transferred to a
Cellophane bag and dialyzed. Without oxalic acid the pigment can
not be made soluble b}^ dialysis.
Writing done with prussian blue ink is very fast to light and to
water. It is not easily removed by the usual two-solution ink
eradicator. The ink is commonly sold as ‘‘acid-proof”, but nothing
is said about the ease vuth which alkaline solutions will destroy the
color. iUkalies, even soap and water, decompose the blue and leave
behind a rusty stain of iron oxide, which can be dissolved by treat¬
ment with a dilute acid.
A solution of 10 to 15 g of prussian blue in 1 liter of water has a
satisfactory depth of color. Because bright blue ink is not popular,
sometimes a dye is added to darken the shade and to produce what
is sometimes called blue-black ink. This name is unfortunately
chosen, because it is commonly understood to mean iron gaUotan-
nate ink.
This Bureau has spent but little time in experimenting with dyes
for darkening prussian blue inks, because they are not used by the
Government and are of little commercial importance. Water-soluble
nigrosine (C.I. 865; Sch. 700), when added to solutions of prussian
blue and oxalic acid, soon formed gummy deposits. Three other
black dyes, durol black B (C.I. 307 ; Sch. 265), Columbia fast black
FF (C.I. 539; Sch. 436), and direct deep black RW (C.I. 582; Sch.
463) were tried, but gave poor results. VTien the mixtures vdth
Prussian blue stood in corked glass tubes for a week, there was no
visible settling out of the color, yet they had a curdled appearance
when used as writing ink, with a pen that was supposed to be gold-
plated. It is suggested that those who are sufficiently interested
should either try other black dj^es, or else hunt for a yellow and a
red dye that can be mixed with the solution of prussian blue, with¬
out causing jelling or precipitation. If added in the right propor¬
tions to the solution of blue, the red and yellow vull produce a practi¬
cally black solution.
4. COLORED DRAWING INKS
In telling about carbon inks it was pointed out that black drawing
ink is not a solution, but only a suspension, of carbon in a liquid
vehicle. A few of the commercial colored drawing inks are sus¬
pensions of pigments in a liquid. These pigments are usually dye
Inks
29
“lakes”, formed by precipitating a dye on an inert material, such as
aluminum hydroxide, barium sulphate, or some other compound.
As the lakes are, in general, of greater density than carbon, and are
not in such fine particles at the outset, the problem of grinding them
so fine that they will not settle to the bottom of the liquid is more
serious than with carbon. The few pigment inks that the Bureau
has examined had a decided tendency to settle. Most colored
drawing inks are clear solutions of dyes. They do not have the same
degree of hiding power as the inks which contain the comparatively
opaque pigments, but their good working qualities outweigh the
disadvantage due to their being transparent.
The first Federal specification for colored waterproof drawing ink
gave formulas for making inks of several colors to serve as standards
for fastness to light and water. Although these standard inks were
given what was thought to be an adequate test in the laboratory,
at the Panama Canal Zone, and in a Bureau where a great deal of
drafting is done, two or three of them developed serious faults in
the course of a year. It became necessary to cancel the specifica¬
tion, pending more laboratory work. The present Federal Speci¬
fication TT-1-531 is based upon the work described in this section.
Making a really waterproof ink, which when dry will not be
blurred by accidental wetting, nor by cleaning with a damp cloth,
is something that can not be done offhand. The ink must contain
materials that are soluble in water, but become insoluble when
they dry on paper or tracing cloth. The usual combination is shellac
and borax, together with dyes that have enough affinity for the
shellac to make them resist the solvent action of water. By no
means all dyes are suitable, because many of them can be leached
out of the shellac-borax film.
The development of satisfactory formulas for drawing inks of
seven colors required a great deal more work than had been antici¬
pated. It did not suffice to use a more concentrated solution of
shellac and borax in the same proportions as at first, but it was found
that if the amount of shellac in a liter was increased, the quantity of
borax lessened, and ammonia added to give sufficient alkali for dis¬
solving the shellac, inks could be made that were extremely resistant
to water. This was true if the dyes were such that they were held in
combination with the shellac-borax film. Otherwise there was no
improvement. Only an outline of the work can be given here, and
the reader is referred to the original article describing what was done.^^
A solution was made by digesting, on a steam-bath, 65 g of dry
orange shellac in 500 ml of a mixture of 1 volume of strong ammonia
water (sp gr 0.90) and 4 volumes of distilled water. When the shellac
was dissolved, the solution was cooled and extracted four times with
a mixture of ethyl ether and petroleum ether, to remove the insoluble
waxy component of the shellac. The solution was analyzed to
determine its content of shellac, and it was then diluted with enough
water to bring the content of shellac down to 50 g in 500 ml. To the
still slightly ammoniacal solution 1 g of phenol and 3 g of crystallized
borax were added. No attempt was made to bleach the shellac solu¬
tion, because it was found that the coloring matter in tlie unbleached
solution did not affect the hues of pale dyes enough to be noticeable.
■7 E. AV. Zimmerman, Colored waterproof drawing inks. Ind. Eng. Cham, "ii, 1033 (1933).
30
Circular oj the National Burecm oj Standards
This stock solution was used for making the actual inks, by mixing
it with equal volumes of solutions of suitable dyes. Before use, the
ink was filtered to remove any insoluble matter from the dye or that
was not removed from the shellac. In all, 92 dyes were tried, and the
most promising, as regards water-fastness and good color, were made
into inks, and 2 bottles of each were stored for testing their keeping
quality. The best of the 92 dyes, and the weight of each that was
dissoh^ed in 50 ml of water and mixed with an equal volume of the
shellac solution, are given in table 1. The dyes marked ‘^second
choice” made ink that did not keep quite so weU as that made vfith
the ^Trst choice” dyes. Yet all 16 dyes made inks that were in good
condition at the end of 2 years, with the exception of 1 of the 2 samples
made with crystal violet. In this the color was destroyed by a
hea^w growth of mold, but the duplicate sample was in good condition.
Table — Dyes for drawing inks
FIRST CHOICE
Name of dye
Colour
Index
number
Schultz
number
Gram
Erythrosin, yellowish _ _ _ _ _ _
772
591
79
0. 5
Brilliant orange R... . _ _ _ _ _ _
78
. 6
Chloramine vellow _ _ . . _ _ _ _ _ _
814
617
. 4
Brilliant miliiQg green B _ _ . _ _ - . .. _ . .
667
503
1.2
Wool blue (? extra. . . . . . . .. _ _ . .. ..
736
565
.5
Iv' ethyl violet B. ... .. _ _ _ _ _ _ .
680
515
.5
Benzamine brown 5GO .. . . . . .. _ _ .. ..
593
476
.8
SECOND CHOICE
Crocein scarlet SB . . . . . .. _ .
252
227
0.5
Benzopurpurine 70B _ _ _ _ _ .. _ _ . _ _
495
405
.6
Orange R _ _ _ _ _ . . .
161
151
.4
Metanil yellow . . . . _ _ ... _ _ .. _
138
134
.8
Thiazol yellow _ .. ............ . . . .
813
198
.8
Malachite green . . _ _ .. _ _ ...... _
657
495
.8
New methylene blue N _ _ . . _ . . _ _ _ .. . .. ..
927
663
.4
Crystal violet _ _ _ _ _ . . . . _ _
681
516
.4
Benzo brown G _ _ .. . _ ..... . ......
606
485
.6
5. SHOW-CARD INKS
The Bureau never has occasion to test show-card inks but is now
and then asked how to make them. A few of the formulas to be
found in books are given here.
Many of the formulas are based upon a solution of sheUac and
borax similar to that described in the preceding section. The solu¬
tion could be made in the same way as for colored drawing inks,
but the extraction vfith the mixture of ethers could be omitted and
the solution simply filtered or perhaps strained through muslin or
other closely woven material. Before being filtered, the solution is
clouded with extremely fine particles of the insoluble waxy component
of the shellac. This tends to clog the pores of the filtering medium,
so that after a time the liquid runs through very slowly. There will
be less trouble from this cause if the solution is allowed to cool first,
because the wax is sticky when hot. To make ink with this solution,
dyes of the desired colors are dissolved in it but in larger amounts than
suffice for drawing inks. Because the requirements for a showcard
ink are less exacting than for a drawing ink, other dyes than those
named in the preceding section can be used.
Inks
31
Instead of dissolving dyes in the shellac-borax solution, dye lakes
or other pigments could be suspended in it. First, the pigments
would have to be ground very thoroughly with enough of the solu¬
tion to make a thin paste, and tliis would then be thinned to a good
working consistency by adding more of the solution. Inks made
in this way would have better hiding power than the clear solutions
of dyes, but it is an open question whether this advantage would
pay for the extra trouble, unless the grinding is done with an ink
mill. Grinding by hand is slow and inefficient, and unless it is done
thoroughly the resulting ink will not be smooth but lumpy.
A formula that is typical of those found for black ink is: Dissolve
16 parts by weight of asphaltum and 18 parts of Venice turpentine
in 50 to 60 parts of turpentine, and add 4 parts of lampblack.
6. HECTOGRAPH INKS
The hectograph is a simple device for making a moderate number
of facsimile copies of a letter or drawing. The original is pressed
down upon a special surface or pad composed of gelatin (or glue)
and glycerin, or of clay and glycerin. The pad absorbs part of the
ink of the original and can then be used for printing upon other
sheets of paper. ‘‘Hectograph’’ means “ hundred writing”, but this
name seems exaggerative.
Hectograph ink must contain a large proportion of a dye that has
good color strength. With the idea, afterwards abandoned, of writ¬
ing a Federal specification for this ink, numerous samples were made
and tested, in order to find out which dyes would give the largest
number of good copies. The inks were made by a formula obtained
from the U. S. Government Printing Office, except that in it acetone
was substituted for alcohol. The formula, in parts by weight, is:
Acetone _ 8
Glycerin _ 20
Acetic acid, 28-percent, commercial _ 10
Water _ 50
Dextrin _ 2
Dye _ 10
If the unit of weight is 1 g, this v»dll make a little more than
90 ml, or about 3 fluid ounces of ink.
The dextrin is first dissolved in the water, which must be heated,
but need not be boiled. Care must be taken not to char the dextrin
at the start, when it clings to the bottom of the vessel in a sticky
mass. It is safest to heat the mixture by setting the container in a
vessel of hot v/ater. When a clear, or nearly clear, solution is ob¬
tained, cool it and add the other liquids. Acetone is combustible,
and quite volatile, but the amount in the ink is not dangerous. If it
should be poured into the hot solution, there would be a brisk boiling,
and much of the acetone would be lost.
In the tests referred to, the following dyes were selected as giving
the greatest number of copies of satisfactory intensity. The best of
all is methyl violet B (C.I. 680; Sch. 515). Crystal violet (C.I. 681;
Sch. 516) is nearly as good. For red ink, rhodamine B (C.I. 749;
Sch. 573) was selected, with fuchsine, or magenta (C.I. 677; Sch.
512), as second choice. Fuchsine is so slightly soluble that only 3.5
parts of it, instead of 10, could be dissolved in 90 parts of the solvent.
32
Circular of the National Bureau of Standards
For green and blue, emerald green (C.I. 662; Sch. 499) and victoria
blue B (C.I. 729; Sch. 559) ranked first, with malachite green (C.I.
657; Sch. 495), and soluble blue (C.I. 707; Sch. 539), second.
Yfith a clay-glycerin pad and inks made with these dyes, it was
possible to get at least 30 copies in which the strokes of the pen were
unbroken, and numerous other copies that were easily legible, though
with more or less broken lines. With some of the numerous dyes
tested, this could not be done.
There is no truly black water-soluble dye, for dilute solutions of ail
the so-called black dyes are blue or purplish. Hecto^aph ink made
with water-soluble nigrosine (C.I. 865; Sch. 700) will make one or
two nearly black copies, but the succeeding ones are of a dingy pur¬
plish gray. It is possible to make black ink by mixing dyes, for
instance green, violet, and yellow, in the right proportions, but be¬
cause the dyes will not be absorbed by the hectograph pad nor by the
sheet of paper in the same proportions, no black copies can be made
after the first one or two.
A bright j^ellow ink made with auramine (C.I. 655; Sch. 493) was
almost illegible, on account of a curious optical effect. Its brightness
was nearly the same as that of the white paper. Because of that and
of its light color, there was no great contrast between it and the
paper, and the copies looked so extremely blurred as to be illegible.
When seen through a blue glass, which made the ink look dark, the
copies were sharp and distinct.
There are simpler formulas for hectograph inks in various books,
but they have not been tested by the Bureau. Because a single name
may be given to two different dyes, the Colour Index and the Schultz
numbers are omitted from the formulas given here. It is probable,
though, that the methyl violet and the fuchsine are the same as those
in the fourth preceding paragraph, and that Hofmann’s violet is
C.I. 679; Sch. 514. In these formulas all parts are by weight.
Blue. — Aniline blue, water-soluble, 1; glycerin, 1; water, 5 to 10.
Indigo-hlue. — Brilfiant green, 3; Hofmann’s violet AlB, 3; glycerin,
1; water, 10.
Green. — Aniline green, water-soluble, 3; glycerin, 2; water, 10;
alcohol, 2.
Bed. — Fuchsine, 1; glycerin, 1; water, 5; alcohol, 1.
Violet.-— NiQthjl violet, 1; glycerin, 2; water, 7.
7. STAMP-PAD INKS
Federal Specification TT-I-556, for stamp-pad ink, gives a formula
for a standard in several colors. The vehicle, or liquid part, consists
of 55 parts by weight of glycerin and 45 parts of water. Equal vol¬
umes of U.S.P. glycerin (sp gr 1.249; 96-percent), and water will make
almost exactly the desired mixture.
Five parts of dye is dissolved in 100 parts of the solvent. The dyes
required by the specification are fuchsine (magenta; C.I. 677; Sch.
512), light green SF (C.I. 670; Sch. 505), soluble blue (C.I. 707;
Sch. 539), acid violet (C.I. 698; Sch. 530), and water-soluble nigrosine
(C.I. 865; Sch. 700). A great variety of other dyes can be used, if
desired.
At ordinary temperatures glycerin practically does not evaporate,
and it is hygroscopic, or attracts moisture from humid au’. This
keeps the ink from dr3fing on the pad, even in winter, when the air in
heated office buildings is of desert aridity. In summer, when the air
Inks
33
is usually of high humidity, the ink tends to take up water from the
atmosphere. There is not enough change in the ink, with variations
of atmospheric humidity, to affect its use.
8. RECORDING INKS
There are numerous kinds of instruments for making continuous
records of temperature, barometric and steam pressures, electric
voltage, etc. The record consists of a line or of a series of dots on a
circular card or a long roll of paper. The instrument may have to
run a long time without attention, so there must be an ample supply
of ink of a kind that will not dry on the pen, if the instrument is
located indoors, nor freeze at outdoor temperatures in winter. For
many years the United States Weather Bureau has used recording
ink made like stamp-pad ink, but with much less dye. The mixture
of equal volumes of glycerin and water is a good ‘‘antifreeze’^ yet
there are parts of the country where it would be solid in winter. To
overcome this difficulty, enough alcohol to keep the ink fluid at any
winter temperature is mixed with the ink. For indoor use, a mixture
of 1 volume of glycerin and 3 volumes of water has been found to
make satisfactory recordmg ink.
The properties of glycerin that make it so valuable in stamp-pad
and recording inks are disadvantages as soon as the ink is put upon
paper. Because the ink must dry almost entirely by being absorbed,
the marks are apt to be “feathered”, or have uneven edges. The
ink spreads in all directions from the actual marks made by the pen,
and does not just go down into the paper. Whether or not the mk
feathers depends upon the paper or card on which the record is made,
and also upon the nature of the dye in the ink. In order to get definite
information on this subject, a number of experiments were made with
inks prepared by dissohdng selected dyes in glycerm and water in the
proportions used for both outdoor and indoor recordmg inks. Inks
were also made with similar mixtures of ethylene glycol and water,
because the glycol is in many respects similar to glycerin, though
somewhat more fluid.
In the first series of tests, 12 acid and 7 basic dyes were made into
38 inks, by dissolving them in each of the 2 mixtures of glycerin and
water already mentioned. Lines were drawn on five kinds of paper,
including three heavy papers, or thin cards, furnished with recording
instruments. The lines v/ere drawn with an ordinary pen, and were
inspected after they had dried naturally. According to their appear¬
ance the lines were rated as N (no feathering), SF (slight feathering),
and F (decided feathering). The results showed that neither the acid
nor the basic dyes could be considered as superior. Of the total 190
lines dravni on the 5 papers with the 38 inks, 10 were rated as N, 108
as SF, and 72 as U. It must be admitted that most of the lines rated
as SF would be considered as quite satisfactory for most uses.
It was thought that direct dyes might make better inks than the
acid and basic dyes. Direct dyes, as their name implies, are taken
up directly by vegetable fibers, without the aid of mordants to fix
them on the fibers by forming insoluble compounds. It was thought
that direct dyes would not spread in the paper when the mixture of
glycerin and water was absorbed, and that therefore the lines made
with the inks would have smooth edges.
>• C. E. Waters, Inks for recording instruments. J. Research NBS 17, 651-655 (1936) RP935.
34
Circular of the National Bureau of Standards
In all, 16 direct dyes were made into 64 inks, by dissolving them in
the 2 mixtures of glycerin and water, and in equally diluted ethylene
glycol and water. The table of results leads to three general conclu¬
sions: Inks made with direct dyes feather much less than those made
with acid and basic dyes ; glycerin at either dilution is a better solvent
than equally diluted ethylene glycol; and, as might have been pre¬
dicted, the greater the proportion of water in the ink, the less is the
tendency to feather.
Feathering Tests of Inks Made With Direct Dyes
Rating
Glycerin-water
Glycol-water
1:1
1:3
1:1
1:3
Total N _
52
79
27
69
Total SF _
21
1
15
11
Total F _
7
0
38
0
It is not necessary to tell the names of the acid and basic dyes that
were used in the tests, but the list of direct dyes may be of interest
to some readers. There are 17 dyes in the list, but the yellow one
was not rated, and does not show in the summary of the results of
the tests given in table 2. Its pale color and brightness made so
little contrast with the paper that it was impossible to decide how to
rate the lines drawn with it. However, it might be used to mix with
other dyes, for instance with blue, to make bright green.
One of the dyes, toluylene orange R, did not make a clear solution,
so it might eventually cause trouble if used for a long time, by grad¬
ually clogging the pen. Another of the dyes, diamine green B, set to
a sort of soft jelly on standing overnight, but this became quite fluid
when shaken, and there was no trouble about drawing the lines with
it. The two green dyes made inks of a rather dull color.
Table 2. — Direct Dyes for Recording Inks
Name of dye
Coloiu-
Index
number
Schultz
number
Benzo fast scarlet . _ _ _ _ _
326
279
Congo red _
370
307
Toluylene orange J? _ _ _ _ _ _ _ _ _
446
362
Toluylene orange 0 - . . . - _ _ _
478
392
Pyrazol orange - _
653
Chloramine yellow _ _ _
814
617
Chloramine green B _
589
470
Diamine green B _ _
593
474
Diamine sky blue FF _ _ _ _ _ _ _ _ _
518
424
Benzo sky blue _ _ _ _ _
520
426
Diamine violet N _ _ _ _ _ _ _ _ _
394
327
Oxamine blue JiR (Erie violet 2B) _ _ _ _
471
385
Bismarck brown R _ _ _
332
284
Benzamine brown SQO _ _ _
596
476
Columbia black EE extra.. _ _ _
539
436
Direct deep blatk RW (Erie black RXOO) . . . . . . . .
582
463
Direct deep black ETV** (Erie black RW) . . . .
582
463
• There were 2 samples of this dye, from the same manufacturer but with different names on the labels.
One gave a deeper color than the other, when dissolved in the same concentration.
Inks
35
When the inks made wdth direct dyes had been kept in corked
glass tubes for 12 weeks, some of the corks were moldy. Of the inks
made with the mixture of equal volumes of glycerin and water, six
had moldy corks. With the more dilute glycerin, 13 of the corks and
3 of the inks themselves were moldy. No mold was seen on any of
the corks that had been wet with the inks made with ethylene glycol,
at either dilution. It might be concluded that the glycol is a better
preservative than glycerin, but this would have to be confirmed by
further experiments.
9. INDELIBLE MARKING INK FOR FABRICS
Federal Specification TT-I-542 for indelible marking ink for
fabrics, gives no formula for a standard ink. Of the numerous
formulas to be found in books, nearly all are for inks made with
silver, or are of the aniline black type.
Although silver ink lool^ black in the bottle, and makes moderately
dark marks on fabrics, its full color must be developed by heat or by
exposure to bright light. Either treatment causes the reduction of
the silver salt in the ink, so that metallic silver is deposited in and on
the fibers. The metal is so finely divided that it looks black. An
old formula, slightly modified, is given in the next paragraph.
Dissolve 5 parts of silver nitrate in its own weight of water, and
then add ammonia water in small amounts until the precipitate that
first forms is dissolved In separate vessels dissolve 3 parts of an¬
hydrous sodium carbonate in 15 parts of water, and 5 parts of gum
arabic in 10 parts of warm water. Pour the three solutions together
and warm gently until the mixture starts to darken. If 1 part equals
1 g, the formula will make about 35 ml, or a little more than a fluid
ounce of ink. This formula has been used for several years by an
institution, in the District of Columbia, where the laundering con¬
ditions are severe, and where other inks that were tried soon washed
out.
‘‘Household’^ ammonia is generally not suitable, because it is apt
to contain other substances than ammonia gas and water. Pure
ammonia water can be obtained from a druggist, and he should be
able to supply sodium carbonate monohydrate, if not the anhydrous
form. The monohydrate contains 14.5 percent of water of crystal¬
lization, so 3.5 parts of it must be used, as the equivalent in alkali
of 3.0 parts of the anhydrous salt. The small additional quantity of
w'ater, 0.5 part, thereby added will m.ake no practical difference.
Metal vessels should not be used for preparing the solution of
silver nitrate, nor for the finished ink. It is simplest to dissolve the
silver nitrate in the bottle in which the ink is to be kept. The salt
is easily soluble, and the water does not need to be heated. The other
two solutions can be poured into the bottle, and if they are warm
enough, no further heating may be required to darken the mixture.
This final step in the preparation of the ink is not necessary, but is
convenient because it is easier to use a dark ink than a pale one.
If the freshly mixed ink does not darken, set the bottle cautiously in
a vessel of warm water.
Because of the chemical action of steel upon silver ink, a gold pen,
or if it can be had a quill pen, should be used with the ink. Lacking
these, use a new steel pen, or a gold-plated one. When the marks
are dry, press them with a hot flatiron, or place them in full sunlight.
36
Circular qf the National Bureau oj Standards
to develop the black color. The marks vdll then be very resistant
to washing, unless the laundry uses too strong a solution of chlorine
as the “bleach'', in which case the silver wdll soon be converted into
silver chloride. This is not very soluble in water, yet sufficiently so
to be washed out of the fabric in a short time.
There are formulas for silver marking inks colored with dyes, but
they seem to have no advantage over black ink. In fact, the silver
in the ink soon makes the marks turn black, and the dye washes out.
Aniline black is extremely fast to light and to washing, so it is
suitable for m.arking ink. Possibly the water-insoluble forms of
some of the black dyes are also used n comm.ercial indehble inks.
The Bureau has never made inks of either kind, and rarely has
occasion to test samples. The formula about to be given is typical
of those for the development of aniline black. The ink consists of
two solutions which are to be mixed immediately before use. In the
formula, all parts are by weight.
Solution A
Copper (cupric) chloride _ 85
Sodium chlorate _ 106
Ammonium chloride _ 53
Water _ 600
Solution B
Gum arabic _ 67
Aniline hydrochloride _ 200
Water _ 335
For use, mix 1 volume of A and 4 volum.es of B. The marks need
not be exposed to sunlight nor ironed to develop their color.
Sodium chlorate is quite different from sodium chloride, common
salt. The chlorate is extremely dangerous when mixed with com¬
bustible substances, because friction or shock may cause the mixture
to explode with great violence. Potassium chlorate is easier to
obtain than the sodium salt, but it can not be used in this formula,
because it is not sufficiently soluble in water. To dissolve 122 parts,
which is chemically equivalent to 106 parts of sodium chlorate,
would require about 1,900 parts of v/ater at ordinary temperatures.
An aniline black ink to be used with a rubber stamp is made as
follows: Grind separately, and as fine as possible, 20 parts of copper
sulphate crystals (bluestone) and 30 parts of aniline hydrochloride
(aniline salt). Mix the two, add 10 parts of dextrin, 5 parts of glyc¬
erin, and then water in small portions until the mass has the right
consistency.
An interesting development of recent years is an invisible laundr}^
ink. Marks made with it can not be seen unless illuminated by a
special lamp that gives a great deal of ultraviolet radiation. The
marks then glow with a phosphorescent light that is in good contrast
with the background of unmarked fabric. The Bureau has not
analyzed the ink, and can give no information about its composition.
10. SYMPATHETIC OR INVISIBLE INKS
Persons who indulge in secret writing for legitimate or nefarious
reasons must have invisible or sympathetic ink. In their chapter on
this subject, Mitchell and Hepworth^^ say that both Ovid (43 B.C.
Inks, Their Composition and Manufacture, 3d ed. p. 286 (Chas. Griffin & Co., (Ltd.), London, 1924).
Inks
37
to A.D. 17) and Pliny (no doubt the Elder, A.D. 23 to 79) tell about
sympathetic inks. The}^ knew of the use of milk and plant juices
for this purpose. When heated moderately the writing turns brown
before the paper begins to scorch, and thus the message becomes
readable. These are but two examples of secret inks that are made
visible by heating. Some of the inks char more easily than the paper,
but others cause the paper to char. In either case the writing turns
brown, or even jet-black.
Without going outside of the home, anybody can get several ma¬
terials for making sympathetic ink that can be developed by heat.
Any of the following substances can be used, though it must be con¬
fessed that some of them are very poor indeed: Alum, soda (either
baking or washing), borax, flour or starch boiled with plenty of water,
a solution of soap or of washing powder, diluted mucilage, milk, lemon
juice.
Ammonium chloride, ‘‘sal ammoniac”, dissolved in 15 to 20 times
its weight of water makes an ink that is invisible, but becomes dark
brown or black when pressed with a hot iron, or held at a distance
above a small flame. This is as good a sympathetic ink as any, is
easy to prepare, and is not dangerously poisonous.
The salts of several metals have long been favorite materials for
sympathetic ink. These salts are not ail colorless when in the solid
form, or in strong solution, but invisible marks made on paper with
very dilute solutions can be developed by suitable means. Among
these salts are lead acetate, ferric sulphate, mercuric chloride (cor¬
rosive sublimate, dangerous to handle and very poisonous), copper
sulphate, cobalt chloride, and nickel chloride. In addition to being
turned brown or black by the fumes of ammonium sulphide, writing
with any of the salts can be developed by heat, and still other means
can be employed with some of them. For instance, if the ink is
made with ferric sulphate, a solution of gallic or tannic acid will turn
the writing black, and potassium ferrocyanide will form prussian blue.
Of the salts just mentioned, cobalt chloride is in some respects the
most interesting. When a solution of the salt in water evaporates
to dryness, the chloride appears in crystals that are red, though not
intensely so. If the solution used as sympathetic ink is so dilute as
to be only of a moderately deep pink, the thin layer of the salt that is
left on paper when the writing dries will not be perceptible. If the
writing is kept for some time m rather dry air, or is warmed slightly,
the cobalt chloride loses most of its “water of crystallization”, and
is then so intensely blue that the writing is visible. Exposure to
moist air, as by breathing upon it, makes the writing vanish because
the blue salt regains water of crystallization and turns red. These
changes back and forth can be repeated many times, but if once the
secret writing should be heated too strongly when warming it, the
chloride will char the paper, and the writing will then be permanently
black.
As a means of developing writing done with a variety of inks, iodine
is interesting. It is preferably used as the vapor given off by the
solid element at ordinary temperatures, though the tincture diluted
with water can be employed. If a thin solution of boiled starch was
used for the writing, iodine will turn it blue. The color disappears
after a time, and more quickly by gentle warming. Writing with a
solution of soap becomes yellow or brown because the soap absorbs
38
Circular oj the National Bureau of Standards
iodine vapor more easily tha.n paper does. This color soon vanishes
because the iodine is so volatile. Copper sulphate and lead acetate
are colored temporarily, while marks made with mercuric chloride
show as w^hite on a background of yellow paper. If the writing was
done wdth distilled water, iodine vapor will color the letters a little
more strongly than the background. The water disturbs the sizing
at the surface of the paper, and thus allows the iodine vapor to be '
absorbed more readily there than elsewhere.
In the examination of a document suspected of containing secret
writing, the first step would be to try the effect of ultraviolet radiation,
which causes many substances to glow with a phosphorescent light
that can be photographed. If this test shows nothing, the next step
would be to warm the paper moderately, and small portions of it
possibly more strongly. This may not bring out anything, but any j
latent writing is not apt to be destroyed by the treatment. Exposure !
to iodine vapor or to the fumes of ammonium sulphide might be next '
in order, or the fumes of ammonia water could be used. If heat and
the various vapors fail, chemical solutions must be tried on selected
small parts of the document. To treat the w^hole sheet of paper with
a reagent that brought no visible result might destroy all chance of
developing the writing. If a chemical solution applied to a small
part of the document shows that there is secret writing, the entire
sheet of paper can be treated with the solution. It is safest to do this
in a letterpress wdth cloths moistened with the chemical solution. In
this way a minimum of water will be used. If the secret message was
written with an easily soluble substance, dipping the sheet into, or
brushing it over with, the solution might dissolve the invisible sub¬
stance and thus destroy the secret writing. In a letterpress, unless
the cloths are too moist, there is little flow of liquid, and the danger
of hopelessly blurring the writing is reduced to a minimum.
11. INKS FOR SPECIAL SURFACES
All the inks so far taken up in this circular have been intended for
use on paper and similar materials, and they are not well adapted to
writing on impervious or oily surfaces, such as glass, porcelain, cellu¬
loid, metals, or painted surfaces. It is true that by going over the
lines repeatedly it is possible to write after a fashion on some of these
materials with WTiting or dravdng inks, but the beha\dor of the ink
shows that it is not suitable for the surface. In this section a few’
formulas are given for inks that will w’ork more or less well on some
special surfaces. Many more formulas can be found in books.
(a) INKS FOR CELLULOID
Trial of two published formulas for inks to be used on celluloid
showed that they would make permanent marks, because they con¬
tained acetone, a solvent for celluloid, so they readily "hook” on it.
The trouble with the inks was that they spread excessively over the
surface, so that wdth a fine-pointed pen the narrowest line that could
be drawn was about one-eighth of an inch wdde.
Some years ago it was found that a commercial solution of bitumen
in coal-tar naphtha could be used for writing on celluloid. It was
necessary to dip the pen into the solution, and then to touch it to
the celluloid without an instant’s delay, or the point of the pen would
Inks
39
become too dry to let the ink flow. The slightly raised letters were
quite resistant to rubbing, though they could be erased by means of
absorbent cotton moistened with benzol.
The success with this solution led to experimenting with solutions
of asphalt in different solvents, and it was found that amyl acetate
(‘‘banana oil”) gave the best results. A solution concentrated enough
to make black marks would not write. A less concentrated solution
made rather sharp lines of a dark-brown color.
(b) INKS FOR GLASS AND PORCELAIN
Some of the inks recommended for writing on glass contain sodium
silicate solution, or water glass, mixed with pigments that are not
changed in color by the alkali in the silicate. Water glass should
not be used if the marks are to be removed later because, when the
solution dries completely, the silicate forms such a strong bond with
the glass that it can not be removed completely without grinding.
A typical formula for ink of this kind is to mix 11 parts of drawing
ink and 1 or 2 parts of water glass.
Dissolve 4 parts of rosin in 30 parts of denatured alcohol. Sepa¬
rately dissolve 4 parts of borax in 50 parts of warm water. Mix the
two solutions and let stand overnight in a loosely corked bottle. Next
morning pour off the clear brown solution, and use it for dissolving
dyes to make inks of the colors desired. The crystalline deposit on
the bottom and walls of the bottle consists chiefly of borax, mixed
with a little rosin. Because this ink has a slight tendency to spread
on glass, it should be used with a fine-pointed pen.
A similar ink can be made by dissolving dyes in a solution of shellac
and borax in water, without alcohol. The solvent is made by heating
nearly to boiling a mixture of 4 parts by weight of dry orange shellac,
1 part of borax, and 150 parts of water. It may take 2 or 3 hours to
dissolve the shellac. The solution must be filtered, preferably after
it has cooled, to remove the insoluble v/axy portion, the orpiment that
settles to the bottom, and the miscellaneous impurities that shellac
always seems to contain. The purplish color of the solution will not
interfere noticeably with the hues of the dyes that are dissolved in
it to make the ink. From 0.5 to 1 g of dye will usually suffice to
make 100 ml of ink. The following dyes are suggested in addition
to those named in connection with waterproof drawing ink:
1 Dya
Colour
Index
number
Schultz
number
Naphtliol yellow _ _ __ _
10
7
Tartrazine (orange in this ink)
640
23
Diamine sky blue FF _ _ _
518
424
Naphthol blue-black S (green-blue) _ _ _
246
217
Benzo cvanine R (verging on violet) __ __
405
336
Durol black B (blue-black) _ _ __
307
265
Nigrosine (purplish or bluish black) __
865
700
It should not be thought that this ink cannot bo washed from
glass. To get such a degree of fastness, water-glass ink or actual
etching must be resorted to.
40
Circular of the National Bureau of Standards
(c) ETCHING INKS FOR GLASS
The Bureau’s Letter Circular LC150, Dry Etching of Glass, gives
a few formulas, and tells in detail how certified burettes and other glass
measuring apparatus are marked. On application to the Bureau,
a copy of the Letter Circular will be sent wfithout charge.
(d) INK FOR ZINC GARDEN LABELS
Ink for labels made of sheet zinc is of interest to many gardeners.
Perhaps the most durable marks are those made with a solution con¬
taining copper, which is precipitated upon the zinc. As the zinc
slowly weathers away, the copper in the marks keeps up an electro¬
lytic action, so that the writing persists as very slightly sunken lines
of black on the zinc. Labels written with this sort of ink have not
been obliterated by exposure to the weather continuously for 5 or 6
years in the climate of Washington. If the writing becomes obscured
by the products of the corrosion of the zinc, ail that is necessary to
restore its legibility is to rub it with the finger to remove the whitish
coating.
It is interesting that unless there is a normal amount of rain to
wash off the corrosion products, the labels are not very satisfactory,
as in a greenhouse where the plants are sprayed at intervals, but where
there is not enough spraying to be the equivalent of periodical rains.
It helps a little to dip the unwritten labels for a few minutes in a 5-
or 10-percent solution of potassium bichromate. This treatment
retards the rate of corrosion where the water rests in drops on the
zinc, and the writing is less apt to be obscured.
A simple formula for the ink is to dissolve 1 part each of crystallized
copper sulphate (‘Tluestone”) and potassium chlorate in 36 parts of
water. In the discussion of indelible marking inks there is a caution
about handling chlorates, which should be read again.
The ink reacts with zinc so that copper is thrown out of solution
in metallic form. A similar reaction takes place with iron, so a steel
pen is not the best for writing with this ink. A gold pen should be
used, if possible, or a gold-plated one. If these were the days when
‘‘penknife” mxant what it says, a quill pen would also be suggested.
Whatever the kind, it should hav^e a fine point, because the ink has a
tendency to spread a little on the surface of the zinc. If the metal is
reasonably clean, it is best to write on it directly, but sometimes it
must be cleaned with sandpaper. After this it should be rubbed with
the fingers, for without the slight film of grease thus applied, the ink
may spread and make only unreadable blurs.
(e) INK FOR BRASS
Instruments makers sometimes give brass a fine mat black finish by
dipping the perfectly clean metal into a solution made by dissohung
copper carbonate in ammonia water. This suggested trying ink
made by dissolving the more easily obtainable copper acetate in
15 times its weight of water, and adding to this enough strong ammo¬
nia water to dissolve the blue precipitate that is first formed. This
makes good black marks on brass, but not on copper, because the
blackening is caused by a chemical reaction between the copper in
the solution and the zinc in the brass. A similar solution of copper
sulphate does not make as black marks as the acetate solution.
Inks
41
(f) INK FOR OTHER METALS
Inks made by the first two formulas for writing on glass and
porcelain are said to be good for use on metals also.
A dilute solution of silver nitrate in water makes black marks on tin,
brass, copper, and other metals. The marks will be still better if
ammonia water is added to the solution of silver nitrate. At first a
black precipitate is formed, but more ammonia will dissolve it. This
solution makes beautifully sharp, black lines. It has the disad¬
vantage that the silver compound formed by the action of the ammo¬
nia is explosive. It seems to be safe so long as it is in solution, but
the solid left behind when the water evaporates is sensitive to shock,
and may explode with great violence if handled roughly. The crusts
that sometimes form around the cork of the bottle in which the solu¬
tion is kept are the explosive compound. If any such deposit is
noticed, it should be rinsed off. To try to wipe it off may cause it to
explode.
For writing on aluminum, the solution of shellac, borax, and dye
that was recommended for glass is quite satisfactory.
(g) TIME-CARD INK
On letter boxes in the smaller towns, the hours at which mail is
collected are written on wliite lacquered cards. The specification
under which the ink is bought requires that it shall be at least as
permanent as ink made as follows: Mix 25 parts of shellac varnish (4
pounds to the gallon), 10 parts of denatured alcohol, and 15 parts of
technical cresol. In this dissolve 5 parts of nigrosine base (C.I. 864;
Sch. 698). This ink can be used for writing on a variety of surfaces,
and would no doubt be found useful for garden labels not made of
zinc.
IV. THE TESTING OF INKS
An essential part of each Federal specification for an ink is the
section in which the methods of testing are described. If the specifi¬
cation gives a formula for a standard ink, both the sample and the
standard are subjected to the same tests, and so far as possible on the
same sheet of paper. The chief reason for having a standard ink is
to use it as a basis for comparison in the testing, because any dis¬
similarity between it and the sample becomes apparent at once. If
the sample is a red ink, or a stamp-pad ink, it need not match the
standard in color, unless this has been agreed upon by buyer and
seller. However, the sample must be as satisfactory in working
qualities as the standard, and as fast to light.
1. IRON GALLOTANNATE INK
Both kinds of iron gallotannate ink can be considered together,
because exactly the same test methods are employed for both. The
tests follow naturally from the definition of ink given by Schluttig
and Neumann, which is quoted on an earlier page of this circular.
The definition clearly tells what properties the ink should have, and
the tests are designed to find out whether the sample has these prop¬
erties. In the fifth chapter of their book, their testing procedure is
given, together with detailed explanations of the tests, some of which
are not required by the Federal specifications. Schluttig and
Neumann do not give a corrosion test.
42
Circular of the National Bureau of Standards
Because freshly made, well-settled ink should be clear, the first
step in the examination of a sample is to allow the ink to remain un¬
disturbed for 24 hours. If the sample is in several small bottles,
which together contain the pint of ink called for, the contents of the
bottles are poured into a single one of a suitable size. If the sample
is concentrated ink, it is diluted with the requisite volume of water
and thoroughly mixed. Powders and tablets are dissolved in water.
In any case, after 24 hours, the bottle, which has not been disturbed
meanwhile, is held up against the light, and is slowly tilted to see
whether any sediment is at the bottom. There should be at most
only traces of sediment. Very muddy ink can be rejected without
making any tests.
If the ink passes this inspection, the test for keeping-quality is
started next, because it takes 2 weeks to complete. In two similar
clear glass vessels, for instance, crystallizing dishes 50 mm (2 inches)
in inside diameter and about 45 mm {ly^ inches) deep, are placed 25-ml
portions of the sample and of the standard. The dishes, loosely
covered, are kept where they will be in diffused daylight, but never
in direct sunlight, for 2 weeks. At the end of this time, the sample
should show no more surface skin than the standard, nor more deposit
on the bottom and walls of the container.
The iron content of the sample is determined in a 10-ml portion,
by any suitable analytical procedure. The amount of iron in 100 ml
of copying and record ink should not be less than 0.58 g, nor more
than 0.70 g. For writing ink the limits are 0.29 and 0.35 g.
Streaks are made side by side on white bond paper with the sample
and the standard. The sheet of paper, 8 by 10>^ inches, or of any
other convenient size, is pinned to a board or clamped to a pane of
glass, and held at an inclination of about 45°. When arranging the
sheet of paper, it is desirable to place it so that the ink streaks will
be made in the “machine direction”, or lengthwise of the long band
in which it comes from the papermaking macliine. Although the
fibers of paper seem to be arranged haphazardly, there is a definite
difference in some of the properties of the sheet in two directions, one
the machine direction and the other at right angles. If a piece of
paper about an inch square is laid upon water, in a moment or two
it starts to curl up on opposite sides, thus making a shallow trough.
The axis of this trough is parallel to the long direction of the paper
as it comes from the papermaldng machine. This should be the
direction in wTich the ink is made to flow when making streaks. If
the ink flows across the machine direction, the paper wull become
wrinkled across the streaks, which will then be unevenly colored.
Measured portions, of about 0.6 ml each, of ink are allowed to flow
down across the sheet of paper. The ink is measured in a pipette
made of glass tubing of 3.5 mm (Ys inch) bore, and 250 mm (10 inches)
long. A mark is etched or scratched 60 mm {2y inches) from one
end of the tube. These are only approximate measurements, because
the exact volume of ink is of no great importance. The ends of the
tube can be fire-polished, but they should not be constricted. Ink is
drawn up to the mark, and kept from flowing out by pressing the tip
of a finger against the upper end of the tube. While holding the
tube vertically, its lower end is held against the upper edge of the
inclined sheet of paper, the finger removed, and the ink is let flow
out aU at once and down across the paper. One or two more streaks
Inks
43
are made with the sample ink and then, with another pipette, streaks
of the standard, close beside those of the sample. The sheet of paper
is left in position until the streaks are dry, and is then put where it
\^ill be in diffused daylight, not in direct sunlight.
It is not necessary, according to the specification, that the freshly
made streaks of the sample and of the standard shall be of exactly
the same color, though they should be equally uniform in color.
They should be of about the same shape and width, because these
features indicate that the two inks are about equally fluid — an
exact measurement would be a waste of time. The streaks made
with the sample should show no more evidence of striking through
the paper than do those of the standard.
After being kept in diffused daylight for a week, the streaks of the
sample should be as intensely black (in reality blue-black) as those
of the standard. The bottom half inch of the sheet is cut off and
discarded. Then five strips, each about an inch wide, are cut from
the lower end of the sheet. One strip is soaked in distilled water for
24 hours, the next strip is kept away from intense light and laboratory
fumes. The third strip is soaked in a mixture of equal volumes of
denatured alcohol and water for 24 hours. The fourth strip is also
put away for later comparison, and the fifth is exposed at a distance
of about 10 inches from a glass-enclosed carbon arc for 48 hours.
When an arc lamp is not available, the test can be made by exposing
the writing to direct sunlight, on the outside sill of a window facing
south, but about double the number of hours will be required. In
these three tests the sample should retain its color as well as does the
standard. The comparison is made easier with the aid of the two
strips, the second and fourth, that were set aside.
Because of the temptation for the manufacturer to increase the
amount of free mineral acid in his ink, so as to delay the deposition
of sediment, a test of the corrosive action of the ink upon steel pens
is made. This test is of no interest to the milhons of users of fountain
pens, because gold is not attacked by the acids that are in ink. The
millions of users of steel pens must be looked out for. It is incredible
how many steel pens are sold each year. In the fiscal year beginning
July 1, 1932, the Post Office Department alone asked for bids on
5,212,800 steel pens.
The amount of metal dissolved from the pens is a rough measure
of the acidity of the ink, and its determination may have some value
in preventing the use of excessively acid ink for writing that must
be kept for a long time.
To make the corrosion test, take two new steel pens, from the same
box, for each of the samples, and two for the standard ink. Rinse
the pens with alcohol, then with ether, and dry them in an air-oven
at 105° C (221° F). Yfeigh each pair together to the nearest
milligram. Because the preliminary washing is to remove the oily
film from the metal, the pens should afterwards be handled with
forceps. Immerse each pair of pens in 25 ml of ink, contained in a
small beaker or flask, taking care not to have them “nested” together.
After 48 hours, remove the pens, clean them with water and by
rubbing to remove the tightly clinging deposit, rinse them with
alcohol, and dry at 105° C. Again weigh, and if the loss in weight
of the pair of pens in the sample is greater than the loss in the stand¬
ard ink, the tests should be repeated with both inks and with new
pens, to check the first results.
44 Circular qf the National Bureau oj Standards
Ink thnt contains oxalic acid usually forms on the pens a yellow
crust of ferrous oxalate, which is not easy to remove by washing
and scrubbing. To get rid of the crust, wash the pens with water
and then, with forceps, hold them one at a time a few inches above a
small flame, but do not heat them to redness. As soon as the crust
blackens and ceases to smoke, drop the hot pens into water. It
will then be comparatively easy to clean them in the regular way.
The weight of metal dissolved depends to some degree on the
surface area of the pens. This would not be true if corrosion stopped
as soon as all the free hydrochloric or sulphuric acid was neutralized
by dissolved iron. Kupert showed that when pens are immersed
in dilute solutions of hydrochloric and other acids, ‘^an amount of
iron equivalent to the total acidity of the solution was dissolved within
3 days at most. Then foilov/ed a slow, steady, and very slight loss
due to ‘rusting’ (oxidation by dissolved oxygen or water itself). In
inks, the time required to reach the maximum was slightly greater.
This was followed by a gradual loss which was independent of the
original acid concentration, but of greater extent than the rusting
noted in the experiments with acids. The loss was caused by the
action of the tannic and gallic acids. This observation was con¬
firmed in another experiment in which tannic and gallic acids alone
were used in the solution.”
Rupert also designed a simple apparatus for maldng a corrosion
test, by alternately dipping the pens into the ink, hanging them in
the air to dry, again dipping, and so on as long as desired. By this
method the building up of a deposit on the pens could be observed.
Many persons regard this deposit of dried and more or less oxidized
ink as evidence of corrosion, though it may not be.
The surface area has no direct connection with the weight of the
pens, so the loss in weight is stated in milligrams, and not as a per¬
centage. The test is at best a crude one, and the results of duplicate
determinations do not check one another very closely.
As a rule, ail parts of the pens seem to be equally attacked by the
ink, but now and then a strikingly different type of corrosion is met
with. In this, the attack is chiefly at the edges, including those of
the open slot and of the slits. Pens have been seen with then* central
slits opened to a width of nearly a millimeter (one twenty-fifth of
an inch). This effect, which seems not to have been mentioned in
print elsewhere than in the immediate predecessor of this circular, is
duo to the ink, because both pens are always affected in the same
way and to the same extent in a given ink. The effect has never
been noticed with the standard ink. A possible reason is that the
ink contains an acid that is barely able to dissolve iron, so its action
is limited to the parts of the pens that have undergone the most
severe mechanical treatment. This, of course, is where the metal
has been cut.
20 F. F. Rupert, Ind. Eng. Chem. 15, 489-493 (1923).
21 Mitchell and Hepworth, In the third edition of their book (see footnote 1), say on page 162; “This
method, which the writer [presumably Mitchell] devised some years ago and published in the first edition
of this book, has recently been included in the United States specifications without acknowledgment.”
On page 123 of the first edition of the book (1904) the following is found: “Another practical test is to immerse
a pen in the ink for a given period, and to determine the loss in weight. Thus in the case of the ink re¬
ferred to above we found that a pen had lost 5.18 percent of its weight after being kept in 10 cc of the ink
for a month, whilst the ink itself had become nearly solid.” The germ of the method in the specification
may have come from the book, but at least as long ago as 1907 the test had been developed to its present
form, which differs in several (ietails from the procedure of Mitchell and Hepworth.
Inks
45
Other ways of determining the corrosiveness of inks have been
suggested.^^ The most unexpected is that of Schiuttig and Neumann,
who said:
We can be sure that an ink which darkens as quickly and intensely as the
type [standard] can not contain relatively too much free acid — relatively only,
for according to the quantity of iron salts the permissible degree of acidity will
be greater or less, naturally only within narrow limits.
2. RED INK
Red ink is much simpler to test than blue-black ink. It is examined
for sediment in the same way, and is judged more strictly from that
standpoint, for a manufacturer has no excuse for failing to make a
clear solution of a dye, and the solution when made should remain
clear. Ink that is cloudy when received should be rejected without
being tested, unless perhaps it has been chilled in transit. In that
case it should be allowed to com^e to room temperature, to see whether
it will become clear, though this rarely happens.
Streaks are made on paper, and are subjected to the light-fading
test, but for only 24 hours, instead of 48 as for blue-black inks. In
red ink, pens corrode hardly more than they do in distilled water.
The loss in weight of a pah of pens immersed in red ink for 48 hours
will rarely be as much as 10 mg.
3. STAMP-PAD INK
Stamp-pad inks dry almost entirely by soaking into the paper on
which the impressions are made. That they can not dry to any great
extent by evaporation is evident from the fact that they contain a
large proportion of glycerin, v/hich tends to absorb moisture from the
ah and is practically nonvolatile at ordinary temperatures. It is
used in order to keep the ink from drying on the inking pad.
For testing, small stamp pads are miade by cutting disks of white
felt about 6 mm {){ inch) thick and 38 mm (1^ inches) in diameter.
The easiest way to cut them is by means of a brass tube of the re¬
quired diameter, which is sharpened at one end, like a cork borer.
These disks fit snugly inside of rings cut from brass tubing. A pad
made in this way works quite as satisfactorily as the more compli¬
cated arrangement described in the Federal specification. The pads
are placed upon a pane of glass, partly for cleanliness, but also to
prevent the loss of ink that would occur if they rested upon paper or
unpainted wood. Equal volumes of the sample and of the standard
ink of the same color are placed upon separate pads. Impressions
are m.ade with both inks upon the same sheet of white bond paper,
with a clean rubber stamp. The impressions made with the sample
should* dry as rapidlj as those made with the standard, and should
be as sharp and as intensely colored. Impressions made with each
ink are exposed at a distance of about 10 inches from a glass-enclosed
carbon arc for 24 hours. The sample should fade no more than the
standard.
The inked pads are allowed to stand exposed to the air for 10 days.
At the end of that time, the sample should show no more evidence
of absorption of excessive moisture from the air, or of drying and
caking on the pad, than does the standard.
“ Mitchell and Hcpworth, p. 163 of their third edition; see also Analyst 46, 131 (1921). Schiuttig and
Neumann, p. 77-78. Rupert, Ind. Eng. Obem. 15, 489-493 (1923).
46
Circular oj the National Bureau oj Standards
4. INDELIBLE MARKING INK FOR FABRICS
There is no formula for a standard ink in the Federal specification
for indelible marking ink for fabrics. The tests are intended to repre¬
sent severe treatment in a laundry, and also to find out whether the
ink weakens, or ‘Lenders”, fabrics. The first step is to prepare inked
strips of cloth for the tendering test. Strips 4 inches wide and 36
inches long are cut in both the warp and the filling directions from
suitable closely woven cotton or woolen cloth, whichever the ink is
intended for. Each strip is cut into test pieces 6 inches long. Across
half of the test pieces representing each dhection of the weave, the
ink is applied in a band about 1 inch wide. The remaining test pieces
are left iininked. After 10 days, the breaking strengths of the inked
and uninked pieces are determined in a suitable machine. The break¬
ing strength of the inked fabric must then be not less than 90 percent
of the breaking strength of the uninked fabric.
On other pieces of suitable fabric, marks are made with the ink,
strictly in accordance with the manufacturer’s directions. Some of
the marked pieces are kept for 2 weeks, and are then examined for
any discoloration beyond the limits of the actual marks.
Two or tliree of the marked pieces are put through a series of wash¬
ing tests in solutions prepared as follows:
Soap solution. — Dissolve 7 g of white floating soap and 7 g of modi¬
fied soda (58 percent of sodium carbonate and 42 percent of sodium
bicarbonate) in enough distilled water to make the volum^e of solu¬
tion 1 liter.
Oxalic acid. — Dissolve 6 g of crystallized oxalic acid in 1 liter of
commercial 28-percent acetic acid.
Sodium bisulphite. — Dissolve 5 g of sodium bisulphite and 72 md
of hydrochloric acid of specific gravity 1.11 in enough distilled water
to make 1 liter.
Bleaching solution. — Prepare a solution of bleaching powder, and
dilute so that it contains 1.4 percent of available chlorine. For use,
100 ml of this stock solution is diluted with 1,300 ml of distilled water.
All the washing tests are made with the solutions at 65 to 71° C
(149 to 160° F). The marked pieces are immersed in the soap solu¬
tion for 15 minutes, then rinsed 5 times with distilled water, and dried.
These operations are performed 6 times. The saKie pieces are then
treated similarly, and 6 times each, in the oxalic acid for 10 minutes,
the sodium bisulpliite for 10 minutes, and in the bleaching solution for
5 minutes. The marks must be clearly readable after the 24 washings.
5. BLACK AND COLORED DRAWING INKS
The Federal specification for black waterproof drawing ink depends
chiefly upon practical tests. Lines of dift'erent specified widths are
drawn upon white drawing paper and upon tracing cloth. With the
slit of the drawing pen set at a width of 0.003 inch, it must be possible
to draw five lines, each 6 inches in length, at intervals of 5 minutes,
‘‘without extraneous assistance to promote the flow’ of ink.” With
the slit at a width of 0.012 inch, a set of lines shall be drawm and then,
before the}^ are dry, a second set shall be drawm across them at right
angles. The ink shall be regarded as unsatisfactory if the intersec¬
tions of the lines are not clean and sharp. Lines drawm with the slit
of the pen set at 0.012 inch shall be tested for smudging or blurring,
Inks
47
by rubbing them with a dry fingertip 5 minutes after they are drawn.
These lines shall be further tested by cutting the tracing cloth and
paper into strips, and soaking one strip of each in water, gasoline,
benzene (benzol), and carbon tetrachloride for 1 hour. After drying,
the soaked lines shall show no evidence of bleeding, running, or
smearing. To determine the resistance of the ink to the growth of
mold, a portion of it shall be inoculated with the spores of the common
green mold, and then kept in a dark, moist place, at about 30° C
(86° F) for 14 days.
It is specified that the ink shall not contain synthetic dyes. To
test for them, mix 2 ml of the ink with 20 ml of alcohol to which 5 or
6 drops of glacial acetic acid has been added. The carbon flocculates
in a few minutes, and is then filtered off, washed with a little alcohol,
and dried at room temperature. The clear filtrate from the carbon
shall not have more than a trace of color. A colorless filtrate is only
contributory evidence, not proof, that dye is absent. It may have
been carried down with the carbon and the waterproofing material —
probably shellac — when the ink was mixed with the acidified alcohol.
So the dried precipitate is tested with caustic soda solution, con¬
centrated sulphuric acid, and other reagents commonly used for the
detection of dyes. Any change in the color of the precipitate, or
the production of strong, characteristic colors in the reagents shall
be regarded as proof of the presence of dye m the ink.
Although there is no formula for a standard black drawing ink,
colored waterproof drawing inks are tested, in part, by comparison
with inks made as described in section III, 4. The seven ‘‘first
choice” dyes are used for making the standards.
Lines are drawn on drawing paper and on tracing cloth, "with the
slit of the pen set to widths of 0.003 and 0.02 inch. There shall
be no noticeable difference in the intensity or the shade of color
between the fine and the heavy lines. The dried lines shall not be
blurred nor smudged when they are rubbed with a dry fingertip,
Vfhen soaked, as described for black drawing ink, in water, gasoline,
benzene (benzol), and carbon tetrachloride, the ink shall show no
evidence of running, bleeding, smearing, or bleaching. The ink is
tested for resistance to the growth of mold; and the fastness of the
color, when the lines are exposed to the light of a glass-enclosed carbon
arc, shall be determined. Black ink is so exposed for 48 hours, and
colored ink only 12 hours.
V. APPENDIX
1. WEIGHTS AND MEASURES
Metric weights and measures are so simple in principle and in their interrelations
that they are used in most of the formulas in this circular. As many readers will
prefer ordinary weights and measures, some conversion factors and other j^er-
tinent information are given.
The standard unit of weight in the metric system is the kilogram (kg), or
1,000 g; and the unit of capacity is the liter, or 1,000 ml, which is the volume
occupied by 1 kg of pure water at the temperature of its greatest density, and
under a pressure of a normal atmosphere. According to the original intent, the
kilogram was to have been the mass of 1,000 cubic centimeters (cm^) of water
at its greatest density, and a liter would then have been 1,000 cm^. This relation
was not realized, on account of experimental difficulties, and it is now known
that instead of being 1,000.000 cm^, the actual volume of the liter is 1 ,000.027 cmh
The difference, equal to the volume of a small drop of water, is negligible in most
48
Circular of the National Bureau of Standards
chemical work. Many laboratories still cling to the name cubic centimeter for
the volume called milliliter (ml) throughout this circular.
Water expands when heated, and thus its density, or mass per unit volume,
decreases. The change is small for ordinary temperature ranges, and for practical
purposes not requiring great accuracy, the weight of both 1 liter and 1,000 cm^
can be called 1 kg; and 1 ml and 1 cm^ will weigh 1 g. So if a formula calls for
50 g of water, it will be sufficiently accurate to take 50 ml.
If it is desired to measure other liquids than water, instead of vreighing them,
their specific gravities must be considered. For instance, if 35.4 g of concentrated
sulphuric acid is required, that figure must be divided by 1,84, the specific gravity
of the acid. The volume will be 35.4/1.84, or 19.2 ml. The rule is the same for
liquids lighter than water. Thus, 80 g of acetone, of specific gravity 0.79, will
equal 80/0.79, or 101.3 ml.
Liquids pack perfectly Vvdthout air spaces, so it is safe to measure, instead of
weighing, them. This is not true of solids, because the weight of a powder that
a given measure will hold depends not only upon the density of the solid particles,
but also upon their size and shape, and how closely they are packed together.
The specific gravity of the solid particles is not always the most important factor
in the v/eight of a given bulk of them, as an illustration will show. The oxide of
lead known as litharge has a specific gravity that varies somewhat according to
the method of preparation, but it is usually a little above 9. Some determina¬
tions were made of the weight of a sample of unusually fine-grained litharge that
could be packed into the space of 1 cubic inch (16.39 cm^). Taking the specific
gravity as 9, a cubic inch of litharge in one solid piece v/ould weigh
9X 16.39= 147.5 g. The v^eight of the fine powder that could be packed into the
cubic-inch measure was about 35 g, or 112.5 g less than a solid cubic inch would
weigh. In other words, the air between the particles of litharge occupied
112.5/147.5, or about 76 percent of the cubic inch. The apparent specific gravity
of the powder was 35/16.39 = 2.14.
The conversion factors about to be given are based upon accurate comparisons
of the kilogram and the avoirdupois and the apothecaries’ pounds, and of the liter
and the United States gallon. The avoirdupois pound is 16 ounces, and the
apothecaries’ pound, 12 ounces. These ounces are quite different, as the tables
show. Hence the weights of the two pounds are not in the ratio 16:12, but more
nearly as 12:10.
Metric and Avoirdupois Weights
1 kg = 2.2046 pounds = 35.274 ounces.
1 g= 0.001 kg= 0.0353 ounce= 15.43 grains.
1 poimd = 0.45359 kg= 453.59 g= 7,000 grains.
1 ounce = 28,35 g= 437.5 grains.
Metric and apothecaries’ weights
1 kg = 2.6792 pounds = 32.151 ounces.
1 g = 0.0322 ounce = 15.43 grains.
1 pound = 0. 37324 kg = 373.24 g=5,760 grains.
1 ounce = 31.103 g = 480 grains.
Metric and United States measures
1 liter = 0.2642 gallon= 1.0567 quarts = 33.81 fluid ounces,
1 ml = 0.001 liter = 0.0338 fluid ounce.
1 gallon = 128 fluid ounces = 3.7853 liters.
1 qiiart = 32 fluid ounces = 0.9463 liter = 946.3 ml.
1 fluid ounce = 29.57 ml.
1 liter of water weighs 1 kg, or 1,000 g.
1 gallon of water weighs 8.33 avoirdupois pounds, or 10.12 apothecaries’
pounds.
1 fluid ounce of water weighs 1.04 avoirdupois ounces, or 0.95 apothe¬
caries’ ounce.
If the weight of a gallon of water is calculated from the factors for converting
kilograms to pounds and gallons to liters, or 2.2046X3.7853, the result is 8.345
avoirdupois pounds. This differs from the value, 8.33 pounds, in the table.
The reason is that the factors are determined at 4^ C (39.2° F), at which tem¬
perature the density of water is greatest. So at any other temperature than 4° C,
a measured gallon of water (231 cubic inches) will weigh less than 8,345 pounds.
Inks
49
To find the weight at any temperature, multiply the density of water at that
temperature by 8.345. Suppose the gallon of water is measured at 20° C (68° F).
At this temperature the density of water is 0.99823, and 0.99823X8.345 = 8.330
avoirdupois pounds. For ordinary w^ork at any “room temperature”, as chemists
so often say when the exact temperature is of no importance, 8.33 avoirdupois
pounds is sufficiently accurate.
The British, or imperial, gallon holds 10 avoirdupois pounds of water, and is
therefore 1.200 times the United States gallon; and the latter is 0.833 imperial
gallon. So, for instance, 1 imperial quart is 0.9463X1.200=1,1358 liters; and 1
liter is 1.0567X0.833 = 0.8802 imperial quart.
The figures to the right of the decimal points can not be ignored in making cal¬
culations, but when it comes to actually weighing or measuring the materials for
a liter of ink, even the first decimal figure may be of little importance. At the
same time it must be remembered that kitchen scales are not suitable for weighing
grams, nor a battered quart cup for measuring milliliters. It will not do to be too
haphazard when making the standard ink of a specification.
The use of the conversion factors will be seen from the following example:
Suppose it is desired to make 125 gallons of the standard red ink, which contains
5.5 g of dye in a liter. One g equals 0.0353 avoirdupois ounce, and 1 liter equals
1.0567 quarts. Then 1 g in a liter equals 0.0353/1.0567 = 0.0334 ounce in a quart;
and 5.5 g in a liter will be 0.0334X5.5 = 0.1837 ounce in a quart. This last figure
multiplied by 4 to get the fraction of an ounce in a gallon, and then by 125 to get
the weight of dye for 125 gallons, gives 91.85 avoirdupois ounces, or 5.74 pounds
of dye.
2. EQUIPMENT FOR MAKING INK
The manufacturer of inks will have proper equipment, but whoever makes small
batches at home must put up "with makeshifts, unless he has some chemical glass¬
ware for preparing the solutions, a measuring cylinder or two, and moderately
sensitive scales with small weights. Usually the weights are apothecaries’, but
metric weights can be obtained. Dealers in photographic supplies sell scales for
those who make their own developing and fixing solutions.
If ordinary bottles must be used, instead of laboratory ware, and the solutions
have to be heated, there are two safe ways to go about it. One is to set the bottle
in a deep vessel containing cold, or at most lukewarm, water, and then to pour
in hot water slowly, and not against the side of the bottle. Another way is to
set the bottle in water as before, but to put under it a piece of wire netting or a
spiral of heavy wire to keep the bottle from touching the bottom of the vessel.
It can then be heated over a low gas flame or on a stove. The idea in either case
is not to heat the outside of the bottle too quickly while its contents are cold,
because the expansion due to the heat may so strain the glass that it will break.
The materials will dissolve more quickly if the bottle is swirled or shaken fre¬
quently so as to stir up the relatively concentrated solution at the bottom.
So far as possible, avoid the use of metal vessels for making ink. Iron, especially,
should not be used, because it is acted upon so easily by acids, and may also
cause discoloration of some dyes.
3. DYES FOR MAKING INK
Not all dyes are equally suitable for making inks and in the formulas in this
circular, various dyes have been recommended. In each case the name of the
dye is followed by certain letters and number in parenthesis, as “Soluble blue
(C.I. 707; Sch. 539).” An explanation of these symbols is in order.
There are many more dye names than there are different kinds of dyes because
manufacturers like to use names of their own choice for their products. As a
rule, the more widely a dye is used, the more apt it is to have a great many names.
At least a dozen names have been given to the familiar dj^e, bismarck brown.
This practice of multipUing names is enough to cause confusion, and the situation
is further complicated by the fact that nearly or quite the same name may be
23 Those who prefer multiplication to division can start with the relation 1 quart equals 0.94G3 liter. Then
0.9463X0.0353=0.0334 ounce, as before.
50
Circular of the National Bureau of Standards
given to dyes that differ in chemical composition and structure. Thus, there are
several “fast reds” and “soluble blues.” It is true that some attempt is made
to distinguish between different dyes that have the same name by adding letters,
or numerals and letters, for instance, toluylene orange G and toluylene orange R.
This plan might be better if there were not so many letters to choose from, and
so many possible combinations of them. To give an example, a certain dye has
been called erythrosine, and erythrosine followed by Z), B, J, JNV, or TT^, to say
nothing of seven other quite different names that have been given it. Again,
sometimes a manufacturer will sell the identical dye under two or more brand
names.
To do away with this sort of confusion, two important tabulations of dye
names have been published. The first is Gustav Schultz's “Farbstofftabellen”
(Dyestuff Tables), of which there have been several editions. The second is the
“Colour Index” of the British Society of Dyers and Colourists. In each book the
dyes are arranged in groups, according to the type to which they belong, and are
further classified according to their chemical formulas in each group. The dyes
are numbered seriallv, so any one number applies to only one dye. Thus, 707 in
the Colour Index means a particular dye, one of the soluble blues, and nothing
else. The same dye is no. 539 in Schultz’s book. It will now be understood by
the reader that such a symbol as (C.I. 707; Sch. 639) is the most certain way of
telling the seller just what kind of dye is wanted.
The Year Book of the American Association of Textile Chemists and Colorists
gives the names by which the various types of dyes made in this country are
known by the manufacturers. Usually there are several names for each type,
in which case the year book gives a preferred name. With one or two exceptions,
the dye names given in table 3 are the preferred names in the year book. A few
alternative names are also given in the list. Man;/ more dyes than those in the
list can be used for making inks. The list is given as an aid to readers who would
otherwise have no idea what to ask for.
Table 3.- — Dyes for various inks
RED DYES
Colour
Index
number
Schultz
number
Fast crimson _ _ _ _ _ _
31
42
Azorubine (nacarat S) . . - _ _ _ _ - _ _ _
179
163
Crocein scarlet SB (crocein scarlet MOO) _ _ . _ _ _
252
227
Benzo fast scarlet .. -- _ _
326
279
Congo red . _ _ _ _ _ _ _ _ _
370
307
Benzopurpurine lOB _ _ _ _ _ _ _
495
405
Fuchsine (magenta) _ _ _
677
512
Rhodamine B. _ _ _ _ _ _ _ _
749
573
Erytbrosin, yellowish _
772
591
ORANGE DYES
Crocein orange _ _ _ _ _ _ _
26
37
Brilliant orange i? _ _ _ _ _ _ _ _ _ _
78
79
Orange R _
161
151
Orange TA _ _ _ _ .. _ _ _ _ _
374
311
Toluylene orange i? _ _ . _ _ _ _ _ .. ..
446
362
Toluylene orange G _ _ _ _ _ _ ... . . _ _ _
478
392
Pyrazol orange _ - _ _ _ _ _ _ _ _
653
YELLOW DYES
Naphthol yellow _ _ _ _ _ .. .
10
7
Metanil yellow _ _ _ _ _ _ _ ..
138
134
Brilliant yellow _ _ _ _ _
364
303
Tartrazine— _ _ _ _ _ _ _ _ _ _ _
640
23
Auramine _ _ _ _ _
655
493
Thiazol yellow _ _
813
198
Chloramine yellow _ _ _ _
814
617
Inks
51
Table 3. — Dyes for various inks — Continued
GREEN DYES
Colour
Index
number
Schultz
number
Chloramine green B . _
689
470
Diamine green B .. _
693
474
Malachite green (victoria green) _
667
496
Emerald green. _ _
662
499
Brilliant milling green B _ _
667
603
Light green SF. .. _
670
605
BLUE DYES
Naphthol blue-black <S (agalma black lOB) _ _ _
246
217
Benzo cyanine R _ .. _
405
336
Benzo blue fR _ _ _ _
408
337
Diamine sky blue FF (direct sky blue 6B) _ ..
518
424
426
637
Benzo sky blue (direct sky blue) _
620
Soluble blue (bavarian blue BSF) _
705
Soluble blue . _ ..
707
639
Victoria blue . _
729
659
Wool blue G extra _ _
736
665
659
Methylene blue _
922
New methylene blue iV _
927
663
Indlgotln (indigo; indigo carmine) _
1, 180
1,288
877
968
Prussian blue . _
VIOLET DYES
Diamine violet iV... _ _ _ _
394
327
385
514
615
Oxamine blue 4R (Erie violet 4R) _
471
Hofmann’s violet. _ _
679
Methyl violet B _ _ _
680
Crystal violet _
681
616
630
Acid violet _ _ _
698
BROWN DYES
Bismarck brown R. _
332
284
Benzamine brown SOO _
696
476
Benzo brown G _
606
485
BLACK DYES
Durol black jB . _ _ _ _ _ _ _
307
265
Columbia fast black FF (diamine fast black FF) _
639
436
Direct deep black RW _
682
463
Nigrosine base _ _
864
698
Nigrosine, water-soluble _
865
700
In the two Federal specifications for iron gallotannate inks, the blue dye that
must be used in the standard inks is the particular soluble blue designated as
C.I. 707. According to the Colour Index, this dye is derived from a mixture
of triphenylpararosaniline and diphenylrosaniline. These two dyes are insoluble
in water, but dissolve readily when converted into the mixture of the trisul-
phonic acids, or of some of their salts. Because this conversion is never com¬
plete, the dye always contains some of the disulphonic, or even of the mono-
sulphonic, acids. The acids are not used as such, but are converted into their
sodium, ammonium, or calcium salts. The calcium salts of the mono- and di¬
acids are nearly insoluble in water, and for that reason some manufacturers
make the calcium salts when the dye is for use in ink. For two reasons it is
less profitable to sell the calcium instead of the sodium salts. First, the mono-
and disulphonic acids cannot be sold mixed with the trisulphonated product.
In the second place, considering the atomic weights and the valences, a given
52
Circular oj the National Bureau oj Standards
weight of the acid will yield a somewhat greater weight of the sodium than of
the calcium salt. Schluttig and Neumann used bavarian blue DSF, which is
the sodium salt of the disulphonic acid of triphenylpararosaniline, mixed with
more or less of the monosulphonate.
Both tannin and iron salts are used as mordants in dyeing. That is, they form
insoluble compounds with some kinds of dyes, and thus serve to fix them upon the |
fabric. Because of the composition of iron gallotannate inks, by no means all |
classes of dyes can be used in them. The dye must be of a type that does not i
form an insoluble compound with anything else in the ink. Also, because some- ij
times a factory batch of soluble blue is not sulphonated as it should be, it is |
advisable when buying to specify that the dye must be “for ink.” A reliable !|
dealer will then not supply soluble blue that is satisfactory for ordinary uses, but 1
not for making ink. The new formulas in section II, 5 (e) require exceptionally I
good dye, and there may be some difficulty in getting it. Unfortunately there 1
are no laboratory tests, aside from making ink and testing it, by which it can
be determined whether the dye is of the desired quality. With suitable dye, the :
new inks should show greater stability in the sediment test than the standard |
blue-black writing ink. *
A few dyes that are more or less satisfactory substitutes for soluble blue are I
naphthol blue-black S (C.I. 246; Sch. 217), benzo blue (C.I. 406; Sch. 337), 1
diamine sky blue FF (C.I. 518; Sch. 424), and benzo sky blue (C.I. 520; Sch. 426). |
Acid black N (C.I. 294; Sch. 261), when used at the rate of 3.5 g in a liter, has \
nearly the same shade of color as soluble blue. Two dyes that caused rapid j
deposition of sediment were durol black B (C.I. 307; Sch. 265) and direct deep !
black RW (C.I. 582; Sch. 463). These remarks apply only to the use of the dyes j
in iron gallotannate ink. The dyes have not been tested in iron gallate ink. i
In addition to bavarian blue, Schluttig and Neumann named three other dyes
which they used for matching the exact shades of inks submitted to them for test.
Apparently these inks had a wide range of colors, because they used red, brown,
and green dyes. Their red dye was azorubine, which they called nacarat S
(C.I. 179; Sch. 163). The green dye was guinea green B (C.I. 666; Sch. 502),
vrhich they knew as acid green VBSPo. Their “Kastanienbraun” (chestnut
brown) cannot be found in the Colour Index nor in Schultz’s Farbstofftabellen.
The chestnut brown of the Colour Index is umber, not a dye at ail, but an insoluble
earth used as a pigment.
In making dyes, it is often necessary to salt them out of solution; that is, to
precipitate them by dissolving common salt in a concentrated solution of the dye.
When this must be done, the dye unavoidably contains more or less salt. Many
dyes also are intentionally mixed with salt or some other uncolored substance to
dilute them to the strength with which dyers have long been familiar, because the
formulas furnished by the manufacturers are based upon these diluted dyes.
This is a recognized trade practice that is not to be regarded as adulteration.
For making inks it is preferable to have the concentrated forms of the dyes, and
these should be ordered from the manufacturer, even though they cost more.
4. LITERATURE ON INKS
This circular gives only a few of the very numerous published formulas for inks.
Every public litoary has books in which other formulas can be found, as well as
references to still other books and to articles that have appeared in scientific and
technical journals. There is no dearth of reading matter relating to inks, and the
list given here is by no means complete.
1. A. H. Allen, Commercial Organic Analysis, 5th ed., 5, 205-267 (P. Blakiston’s
Son & Co., Philadelphia, 1927).
2. H. Bennett, Practical Everyday Chemistry (The Chemical Publishing Co.,
New York, N. Y., 1934).
3. H. Bennett, The Chemical Formulary, 3 vol. (D. Van Nostrand Co., New
York, 1933-1936).
4. W. T. Brannt and W. H. Wahl, Techno-Chemical Receipt Book (H. C. Baird
& Co., Philadelphia, 1905).
5. D. Carvalho, Forty Centuries of Ink (Banks Law Publishing Co., New York
1904).
6. Henley’s Twentieth Century Book of Recipes, Formulas, and Processes
(Norman W. Henley Publishing Co., New York, 1928).
7. J. B. Lavay, Disputed Handwriting (Harvard Book Co., Cambridge, Mass.,
1909).
Inks 63
: 8. S. Lehner, Manufacture of Inks (Translated, with additions, by W. T. Brannt,
J H. C. Baird & Co., Philadelphia, 1892).
I 9. C. A. Mitchell and T. C. Hepworth, Inks, Their Composition and Manufacture,
I 3d ed. (Chas. Griffin & Co., (Ltd.), London, 1924).
: 10. A. S. Osborn, Questioned Documents (Lawyers’ Cooperative Publishing Co.,
Rochester, N. Y., 1910).
i 11. J. H. Oyster, Spatula Ink Formulary (Spatula Publishing Co., Boston, 1912).
12. O. Schluttig and G. S. Neumann, Die Eisengallustinten (The Iron-Gall Inks)
(v. Zahn & Jaensch, Dresden, 1890).
13. Scientific American Cyclopedia of Formulas. Edited by A. A. Hopkins
(Munn & Co., New York, 1921).
14. E. Spon, Workshop Receipts (E. & F. N. Spon, London and New York, 1917).
15. N. Underwood and T. V. Sullivan, Chemistry and Technology of Printing
Inks, (D. Van Nostrand Co., New York, 1915).
16. F. B. Wiborg, Printing Ink (Harper & Bros., New York, 1925).
Articles in chemical journals are:
L. S. Munson, The testing of vn'iting inks, J. Am. Chem. Soc. 28, 512-516 (1906).
F. F. Rupert, Examination of writing inks, Ind. Eng. Chem. 15, 489-493 (1923).
I E. W. Zimmerman, Colored vjaterproof drawing inks. Ind. Eng. Chem. 25, 1033
(1933).
C. E. Waters, Blue dye as evidence of the age of writing, Ind. Eng. Chem. 25, 1034
(1933).
A few publications of the National Bureau of Standards relate to inks. The
Federal specifications, formerly issued as circulars of the Bureau, are now parts
of the Federal Standard Stock Catalog, and are no longer distributed by the
I Bureau. Many public, college, and university libraries throughout the country
have the publications of the Bureau, and possibly also the specifications, as well
' as publications from other branches of the Government. Those that are in print
can be bought from the Superintendent of Documents, United States Govern-
i ment Printing Office, Washington, D. C., for the prices stated. Postage stamps
will not be accepted, and money is sent at the purchaser’s risk. Postal money
I orders, or coupons sold by the Superintendent of Documents in sheets of 20 for $1,
i: are accepted. The coupons are good until used. In ordering, the name of the
Bureau as well as the title and number of the publication should be given.
The letter circulars mentioned below are mimeographed, and are not handled
j by the Superintendent of Documents, but orders for all other publications should
I be addressed to him.
I, J. B. Tuttle and W. H. Smith, Analysis of Printing Inks, BS Tech. Pap. 39 (1915).
(Out of print.)
Composition, Properties, and Testing of Printing Inks, BS Cir. 53, 1915. (Out
of print.)
E Inks — Their Composition, Manufacture and Methods of Testing. Cir. BS C95,
1st ed. (1920). (Out of print.)
Inks, Typewriter Ribbons and Carbon Paper. Cir. BS C95, 2d ed. (1925).
(Out of print.)
, Inks, Cir. BS C400 (1933). (Superseded by C413.)
! P. H. Walker, Some Technical Methods of Testing Miscellaneous Supplies.
Misc. Pub. BS M15 (1916). A reprint, with notes and corrections, of Bur.
Chemistry, Dept. Agr. Bui. 109, (1912). The methods differ considerably
' from those of the Federal specifications. (Out of print.)
A. E. Kimberly and B. W. Scribner, Summary Report of Bureau of Standards
I Research on Preservation of Records. Misc. Pub. BS M144 (1934). (Out
‘ of print.)
E. W. Zimmerman, C. G. Weber, and A. E. Kimberly, Relation of ink to the preser¬
vation of written records. J. Research NBS 14, 463-468 (1935), RP779. 5c.
E. W. Zimmerman, Iron gallate inks — liquid and powder. J. Research NBS 15,
35-40 (1935) RP807. 5c.
C. E. Waters, Inks for recording instruments. J. Research NBS 17, 651 (1936)
RP935. 5c.
Dry Etching of Glass. BS Letter Circular LC150. Free.
Carbon Paper and Typewriter Ribbons. BS Letter Circular LC424. Free.
Stain Removal from Fabrics: Home Methods. U. S. Dept. Agr. Farmers’ Bui.
1474 (1930). 5c.
B. L. Wehmhoff and D. P. Clark, Standard Mimeograph Ink and Paper. U. S.
Govt. Printing Office Tech. Bui. 15 (1932). Free.
B. L. Wehmhoff, D. P. Clark, and D. H. Boyce, Newsprint and News Ink, U. S.
Govt. Printing Office Tech. Bui. 18 (1933). Free.
54
Circular qf the National Bureau of Standards
The following Federal specifications are sold separately by the Superintendent
of Documents for 5c apiece:
TT-I-521, Ink; copying and record.
TT-I-528, Ink; drawing, waterproof, black.
TT-I-531, Ink; drawing, waterproof, colored.
TT-I-542, Ink; marking, indelible (for) fabrics.
TT-I-549; Ink; red.
TT- 1-556, Ink; stamp-pad.
TT-I-563, Ink; writing.
Washington, October 2, 1936.
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